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General Chemistry | 213 Chapter Fourteen Organic Chemistry Organic chemistry just now is enough to drive one mad. It gives me the impression of a primeval forest full of the most remarkable things, a monstrous and boundless thicket, with no way of escape, into which one may well dread to enter.— Friedrich Wöhler 1835 F undamentally, organic chemistry is the study of carbon compounds. These compounds are made out of carbon, hydrogen, and oxygen with the occasional addition of nitrogen, chlorine, bromine, phosphorus and sulfur. Even though organic compounds only use eight of the more than one hundred elements found on the Periodic Table, the multitude of compounds made and the manner in which they react are dizzying. You may know someone who has taken organic chemistry and you have probably heard stories about large stacks of notecards filled with thousands of reactions and the hours it takes to memorize them all. The fear of organic chemistry grips most budding science students and it is believed that organic chemistry is something to be survived and not enjoyed. Much of organic chemistry is learning the language of organic chemistry. When an instructor says to use acetone or toluene in a reaction, a structure or formula must come to mind. It could be quite deadly to mix the wrong set of chemicals together. To this end, we will begin our study with organic nomenclature. Nomenclature Nomenclature is a system for naming compounds. In organic chemistry, nomenclature starts with a root name that is based on the number of carbons in the longest continuous chain in the compound. The root name is a prefix, it begins the name, and other things are added to it to clarify precisely how the molecule is put together. Let’s start with the root prefixes. General Chemistry | 214 Number of Carbons 1 2 3 4 5 6 7 8 9 10 Root Prefix methethpropbutpenthexheptoctnondec- The dash found at the end of each name indicates that something must come after the prefix, specifically, a suffix, that will indicate something about the bonding occurring in the compound. Organic chemists organize compounds by their bonding structure. In its simplest form, compounds containing just carbon and hydrogen are broken up into three major bonding groups. Bonding Group All single bonds One double bond One triple bond General Compound Name Alkane Alkene Alkyne Suffix -ane -ene -yne To name a compound you combine the prefix of the root name with the suffix that indicates the type of bond found in the compound. Therefore, compounds like methane, propene, and butyne are all possible. These and other compounds are shown on the next page. General Chemistry | 215 Alkane Name Alkene H H H C Methane H C Alkyne Ethene C H H Name Name H H C C Ethyne H H H H H H C C H H H Ethane H C C Propene C H H H H H C C C H H H H C C H H H Propane H C C H Propyne H H H C C H H H C C H C H H H H H H H Butene C H C H Butyne Alkenes and Alkynes A double or triple bond can be found anywhere within a compound. Based on the simple rules presented thus far, it is impossible to distinguish the difference between the following compounds, H H C H C H2C H3C CH3 C CH3 H3C C H C H H H C H H C CH3 H3C C CH2 H All of these compounds have 4 carbons so generically, these are all some kind of butene but the shape of these molecules and the placement of the double bond changes, so they cannot all have the same name. We can begin to distinguish between them by stating the placement of the double bond. H H C H C H2C 1-butene H3C CH3 C CH3 H H3C C C H 2-butene H H H C H C CH3 2-butene H3C C CH2 3-butene H General Chemistry | 216 This helps, but there is no rule that says that you must start counting from the left, but there is a rule that says that we need to keep our numbers as small as possible. So, if we count from the right, rather than the left, we can see that, counting from the right, 3-butene should have been named 1-butene which makes the first and last compound on our list the same molecule. So what about the middle two compounds both of which are named 2-butene? It is clear that these molecules are not the same since the CH3 groups are in different positions on the molecule. The first molecule has the CH3 groups on the same side of the double bond and if you turn your head sideways you can see that the general shape is “C” shaped. This form of the molecule is called “cis” so the name of our molecule would now be cis-2-butene. H3C CH3 C C H H cis-2-butene The shape of the other molecule is “trans.” This makes sense since a transAtlantic flight is one that goes across the Atlantic. If we start on the first CH3 group we must go across the double bond (trans) to get to the other CH3. This molecule is called, H3C H C H C CH3 trans-2-butene You will notice that there is a dash between the various parts of this name. This is because these are supposed to be one long name, so we connect the various parts with dashes. In similar fashion, we can name alkynes by adding the starting point of the triple bond to the name. General Chemistry | 217 H C C H H C C H H H C H H H 1-butyne H C C 2-butyne C H H Cis and trans do not apply to alkynes since their bonds are linear. Only double bonds and rings can be cis and trans. We will discuss ring structures in a later section. Branching and Side Groups A carbon chain need not be linear. It can have smaller carbon chains hanging off of them. A carbon compound with side groups is said to be “branched.” These side groups have the same root names as those presented earlier (meth-, eth-, etc) but we distinguish between them and the main chain of the molecule by giving the side groups a –yl ending. Therefore, a side group would have the following name. Number of Root Carbons Prefix 1 methyl 2 ethyl 3 propyl 4 butyl Etc. Suppose we had the following compound, H H H H C C C H CH3 H H 2-methylpropane The longest chain of this compound has three carbons and it has all single bonds so its root name is propane. But hanging off the second carbon is a methyl group (circled in red) which must be added to the name. We must tell the reader which side group is attached and where it is attached on the molecule so the name of this compound is 2-methylpropane. General Chemistry | 218 If there are multiple carbon side groups on a compound we must tell the reader where each of them are located and how many there are. In addition, if they are not all the same, then the side groups are put alphabetically into the name of the compound. When multiple copies of the same side group show up in a compound we tell the reader how many are present by using a common prefix used in non-metal/non-metal nomenclature, Number of Side Groups 2 3 4 5 6 Prefix ditritetrapentahexa- It may seem counterintuitive but we must state where every copy of a side group is found and how many are found on a compound. For example, consider the following, H CH3 H CH3 H H C H 1 C 2 C CH3 H 3 C H 4 C 5 H H The root name for this compound is pentane but it also has three methyl groups. They are found on the number 2 and number 4 carbons, but since we must name where EVERY methyl group is found AND the total number present, the name of this compound is, 2,2,4-trimethylpentane So we see that there is a methyl group on the number two carbon, another methyl group on the number two carbon, and one methyl group on the number four carbon. That makes three methyl groups so we say that this compound is a trimethyl compound, in this case, 2,2,4-trimethylpentane. If more than one kind of side group is present then they are placed into the name alphabetically. Consider the following compound, General Chemistry | 219 CH3 H H C CH3 H 1 H C 2 C CH2 H 3 CH3 H C 4 H C H 5 H C 6 H H This compound is six carbons long and has all single bonds, therefore its root name would be hexane. On the number 2 carbon there are two methyl groups and on the number 4 carbon there is a two carbon chain side group called an ethyl group. Alphabetically, ethyl comes before methyl, so the name of this compound would be, 4-ethyl-2,2-dimethylhexane Halogen Side Groups Carbon chains are not the only possible side group on a compound. Halogens are often found as side groups of organic compounds also. To name a halogen as a side group, the “-ine” ending of the halogen is removed and replaced with an “o.” Side Group Halogen Name Name F Fluorine fluoro Cl Chlorine chloro Br Bromine bromo I Iodine iodo As before, if we have multiple versions of the same halogen on a compound then we number them and add di-, tri-, tetra-, etc., as appropriate. We also write them alphabetically along with all of the other side groups. Adding chlorine and bromine to our previous compound we would have, CH3 Cl H C 1 CH3 H C 2 C CH2 H 3 C 4 C H 5 C 6 H H CH3 Cl H Br H 5-bromo-1,3-dichloro-4-ethyl-2,2-dimethylhexane General Chemistry | 220 Multiple Double Bonds and Side Groups When a compound has more than one double bond we must indicate where each double bond starts and state whether the double bond is cis, trans, or neither cis nor trans. For example, The first double bond is neither cis nor trans but the second double bond is cis. When cis and trans are used, they always start the name. When multiple double bonds are present, you must use di-, tri, and tetra- to indicate the number. In this case, since this compound has two double bonds, it is a diene, specifically, 1,3pentadiene. But, one of the bonds is cis and this must be added to the name. You do not need to indicate which of the bonds is cis (do not write 3-cis). Any knowledgeable reader will understand that the cis only applies to the double bond starting on carbon number three. Therefore the full name of this compound is, cis-1,3-pentadiene Ring Structures Alkanes are not limited to linear chains of carbon compounds, they can also be rings. When an alkane is in a ring, the prefix “cyclo” is added to the name. For example, consider the names of the following structures, cyclopropane cyclobutane cyclopentane cyclohexane It might seem odd to write some of these structures this way, particularly cyclobutane which is obviously a square but has been drawn here like a General Chemistry | 221 diamond. The reason for this is that, when the hydrogen’s are added to the compound, we attach them using vertical straight lines; cyclopropane cyclobutane cyclopentane cyclohexane This has the added advantage of helping to indicate the shape of the molecule. cyclopropane cyclobutane cyclopentane Above the ring cyclohexane Below the Ring The same information can be conveyed by using dashed and solid triangular bonds. Dashed triangular bonds are always pointing away from you and solid triangular bonds are always pointing towards you. So, in similar fashion, we could write, cyclopropane cyclobutane Sometimes using triangular bonds is the most convenient way to indicate the shape of a molecule. For example, in cholesterol, some of the bonds are above the ring, others are below the ring, and others are not on a ring but are still pointing away from you. It is inconvenient to attempt to draw this large molecule in such a way that will make these bonds easily shown. Using triangular bonds accomplishes this task. cyclopentane cyclohexane H H H H H H H Cholestrerol H General Chemistry | 222 Cis and Trans in Rings If drawn improperly, it is impossible to know the spatial relationship between side groups attached to rings. It is best to draw the bonds straight up and down or use wedges and dashed lines to indicate up and down on a ring. The reason this is important is that rings, like alkenes, can be cis and trans. For example, consider the following molecules, When both chlorines are on the bottom (or top) of the ring, they are cis to each other and when one is on top and the other is on the bottom then they are trans to each other. The bonds in the ring behave like a double bond. They keep the atoms rigidly held into position which allows the molecule to be either cis or trans. Even so, when more groups are added to a ring or across a double bond it can get difficult to determine whether a molecule is cis or trans. For example, H H Br H I Cl I Cl Br bromoiodo-2-chlorocyclopropane Br C Cl H C I C I C Cl bromoiodo-2-chloroethene Br General Chemistry | 223 We can rightfully ask, which of the compounds has the name listed below it? The answer is that both molecules have this name but in organic chemistry that is impossible. Two molecules with different structures cannot have the same name so there must be a way to tell the difference between them. Unfortunately, cis and trans does not help us because it doesn’t tell us which atoms we are choosing to be cis and trans. We must use another kind of nomenclature to describe the differences between these molecules. We will use E, Z nomenclature. E, Z Nomenclature You use E and Z when cis and trans fails to properly identify similar groups. The method relies on the Cahn-Ingold-Prelog ranking system to determine whether a compound is cis-like (Z) or trans-like (E). Cahn-Ingold-Prelog Ranking System To rank the groups using EZ nomenclature the Cahn-Ingold-Prelog ranking system is used. This system ranks the attachments one atom at a time until a difference is found. The group that is larger FIRST, wins! For example, consider the two side groups; Main Chain H F H C C C H H H 1 2 3 Higher Rank H Main Chain H H H C C C H H H I 1 2 3 Lower Rank Starting at the main chain you go to carbon number one of the side chain. Doing so you will find that both side groups have a CH2 attached to a carbon. Thus far, both sides groups are equal. Moving on to carbon number two the side group on the left has a hydrogen, a carbon and a fluorine attached to it while the one on the right has two hydrogens and a carbon. Since fluorine is larger than a hydrogen, the left side chain is ranked higher than the one on the right even though the one on the right is bigger since it has iodine on it. BUT, since you got to the fluorine (on carbon #2) before you got to the iodine (which was on carbon #3), the left side group wins. So rank has nothing to do with overall size; it has to do with the size of the group that you come to first. General Chemistry | 224 Using this system we can resolve our problem with the nomenclature of the molecules shown above, H I H Br Cl Br Cl I E H C H C Cl Z trans-like I C Br E bromoiodo-2-chlorocyclpropane C Cl trans-like cis-like Br I Z cis-like bromoiodo-2-chloroethene If we draw an imaginary line down the middle of the bond connecting the two carbons of interest we can rank the constituents attached to each carbon atom. Chlorine is bigger than hydrogen and iodine is bigger than bromine. By circling the large of the two constituents we can see that the first molecule in each group looks “trans-like” and the second molecule looks “cis-like.” We label these E and Z respectively and add the designation to the name. H I Cl Br H Br Cl I H E-bromoiodo-2-chlorocyclpropane Z-bromoiodo-2-chlorocyclpropane I C E-bromoiodo-2-chloroethene C Cl Br H Br C Cl Z-bromoiodo-2-chloroethene C I You will notice that the E and Z designation resolves the problem of identifying the spatial orientation of the atoms on these molecules. Unfortunately, our problem is not completely resolved. The cyclopropane molecules shown above are not uniquely identified by using the E, Z designation. There are two possible General Chemistry | 225 configurations for both E and Z bromoiodo-2-cyclopropane. These are shown below. H I I Cl Br Br Cl H Br Br H Cl I I H E-bromoiodo-2-chlorocyclpropane Z-bromoiodo-2-chlorocyclpropane Cl Based on the information presented thus far, both pairs of molecules would share the same name but are actually different molecules. You will notice that the left hand molecule and the right hand molecule in each pair are related to each other by being non-superimposable mirror images of one another. To provide each molecule with a unique name we must find a way of indicating to the reader which molecule is which. We do this by using a new naming convention called an R, S configuration. Optical Activity Many molecules are related to each other by being mirror images of one another. When molecules are related in this way they are said to show “handedness”, that is, they are related to each other like your right hand is related to your left. To discover which molecule is left handed and which one is right handed we use the Cahn-Ingold-Prelog method of ranking side groups to decide. Molecules that show handedness all share one thing in common, they all have at least one carbon with four different things attached to them. These molecules are mirror images of each other, but they are not the same. You cannot convert one into the other without breaking a bond. General Chemistry | 226 Molecules that are non-superimposable mirror images of one another are known as enantiomers. The carbon to which the four groups are attached is known as the “chiral” carbon and molecules that have handedness are said to be chiral or have chirality. Chiral molecules have another unique property; they cause light to rotate as it passes through their solution. As a consequence, another way of saying that a molecule is chiral is to say that the molecule shows optical activity. The only way to tell how much light rotates as it travels through a solution is by direct observation. A polarimeter is used for this purpose. A light source sends light through a polarizing filter which causes the light to travel only one direction. As this light passes through a 1 decimeter long tube of solution, the light begins to twist. At the other end there is another polarizing filter that is moved until the maximum amount of light is observed as it passes through the solution. The difference in the angle between the first filter and the second filter is the specific rotation of the organic compound. Many things effect the amount of rotation. The concentration of the solution, temperature, length of the tube, and even the wavelength of the light all play a role in determining the amount of observed rotation. As a consequence, an equation is used to normalize all of these factors and determine the specific rotation of the molecule, General Chemistry | 227 [ߙ]்ఒ = ݈݁݃݊ܽ ݊݅ݐܽݐݎ ݀݁ݒݎ݁ݏܾ ݐܽℎ ݈݁݊݃ݐℎ ݅݊ ݀݁ܿ݅݉ ݁݃ ݊݅ܿ݊ܿ × ݏݎ݁ݐ/݉ ܮ Where T is the temperature (usually, 20°C) and λ is usually the 589 nm line of the sodium spectrum which is also known as the “D” line. The sign of the rotation is always given as either positive (+) or negative (-). So, if the observed angle of a 0.2 g/mL solution in a 1 decimeter tube is +1.3° at 20°C, then the specific rotation would be, +1.3° [ߙ]ଶ = +6.5° = 1 ݀݉ × 0.2 ݃/݉ ܮ Unfortunately, the actual rotation of a molecule, or even the direction of rotation cannot be easily predicted based solely on the molecules structure, so a convention called R,S configurations are used to determine whether a molecule is right or left handed. This convention does not predict optical rotation and there is no direct connection between an R, S configuration and the actual rotation of a molecule. An R, S configuration only helps us to draw optically active compounds and not predict how they rotate light. R, S Configurations The molecule, 2-butanol, has two versions, a right handed and a left handed version. These two are optical isomers of each other and are related by being mirror images of one another. To distinguish between them we will use the Cahn-Ingold-Prelog ranking system to rank the four groups attached to the central carbon atom. In this case, the oxygen, with a mass of 16, has the highest rank (1). The next highest is the –C2H5 group (2) and then the –CH3 (3). Finally, hydrogen has the lowest rank (4) of the four groups attached to the central carbon. General Chemistry | 228 The system works best if the group of lowest rank is pointing away. In this case, hydrogen is the lowest ranking group and it is pointing away as indicated by the dashed line. Starting at the OH group (and ignoring the attachment of lowest rank, the hydrogen (4)) we move clockwise until we get to the C 2H5 group and then continue clockwise to the CH3. This clockwise rotation indicates that this molecule is right handed and is labeled R which stands for recto, the Latin word meaning “right.” The optical isomer is S-2-butanol. The “S” stands for sinister, the Latin word for “left.” If the attachment of lowest priority is not pointing away then the actual rotation will be the opposite of how it appears. For example, if we draw 2-butanol so that the hydrogen is pointing towards us, by following the ranking of the attachments, the molecule appears to S. But, we are looking at the molecule wrong. The eye in the picture below is looking at the molecule from the right direction, where the hydrogen is away. From that perspective, the red arrow is moving clockwise or R. So although the molecule looks S to us, it is actually R, so this molecule is R-2-butanol. General Chemistry | 229 We can now resolve the problem that we had earlier with the cyclopropane molecule. Earlier, we called these molecules E-bromoiodo-2-chlorocyclopropane but found that we could draw two versions of this molecule with the same name. One of these molecules is shown above but drawn twice to show the rotation around the two different chiral atoms in the molecule. On the left, the carbon with the blue dot has four different groups of atoms attached to it. In order of their rank, this chiral carbon is attached to, 1: Chlorine 2: A carbon with iodine and bromine 3: A carbon with two hydrogens 4: A hydrogen atom The hydrogen has the lowest rank. It is on the top of the molecule and by convention, this means that it is pointing towards us. Therefore, the rotation that we see is opposite to the actual rotation so, although the rotation appears to be S, it is actually R. This carbon atom is R. In order of rank, the chiral carbon on the right molecule is attached to, 1: Iodine 2: Bromine 3: A carbon with hydrogen and chlorine 4: A carbon with two hydrogens In this case the carbon with two hydrogens has the lowest rank and because of the orientation of the molecule, it is pointing away from us. Therefore, the rotation that we see is correct. The rotation is clockwise. This carbon atom is R. Therefore, this molecule is R, R-bromoiodo-2-chlorocyclopropane. The mirror image of this molecule would be S, S-bromoiodo-2-chlorocyclopropane. By General Chemistry | 230 using R, S nomenclature rather than E, Z, the problem of their nomenclature has been resolved. Labeling Carbon Atoms Sometimes it is useful to discuss molecular structures by referring to a carbon atom by the number of other carbons attached to it. A carbon atom can have as many as four other carbon atoms attached to it or as few as none. Each of these conditions is given a special name, they are, These designations sometimes show up in the names of compounds. For example, 2-butanol is sometimes known as sec-butanol since the OH group is on a secondary carbon. Also, it is very common to refer to 2-chloro-2methylpropane as t-butyl chloride, since the chlorine is on the tertiary carbon and there are four carbons in the structure. A molecule usually has several different kinds of carbons in its structure. For example, the molecule below has five 1° carbons, one 2° carbon, one 3° carbon and one 4° carbon. General Chemistry | 231 Functional Groups Hydrocarbons are not particularly reactive compounds. Since carbon and hydrogen have very similar electronegativities (2.5 and 2.1 respectively) the bonds between them are almost entirely covalent. This renders these molecules essentially inert. If a highly electronegative atom like oxygen, nitrogen, or a halogen is bonded to a hydrocarbon, these atoms polarize the molecule and this gives the molecule a site where reactions can take place. With the addition of an electronegative atom, hydrocarbons become reactive and can do interesting chemistry. The kinds of chemistry that can be done depend on the nature of these additional electronegative atoms. These atoms convert an otherwise inert hydrocarbon into a functioning molecule capable of all kinds of chemistry. They are, therefore, called functional groups. Name Functional Group Alkyl Halide Alcohol Amine Ether Aldehyde Aromatic (Arene) Name Acid Halide Acid Amide Ester Ketone Functional Group Alkyl Halides As stated previously, hydrocarbons, that is, alkanes, are essentially inert. To make them reactive an electronegative atom must be added to the compound. While it is possible to add oxygen to a hydrocarbon, doing so is not really an organic reaction, it is a combustion. Burning a hydrocarbon is not very useful procedure toward organic synthesis. General Chemistry | 232 Hydrocarbons are so unreactive that very few reactions are known to occur with them. The most important of these reactions is called Free Radical Halogenation, a reaction that converts a hydrocarbon into an alkyl halide. .. ௧ CH4 + Cl2 ሱ⎯⎯⎯⎯⎯ሮ CH3Cl + HCl The product of Free Radical Halogenation is an alkyl halide (CH3Cl) and the hydrohalide (HCl). Free Radical Halogenation is not restricted to chlorine; bromine is often used and is usually preferred. The reactivity of the halogens follows their reactivity on the Periodic Table. On a relative scale the reactivity’s are, Fluorine (108) > Chlorine (1) > Bromine (7×10−11) > iodine (2×10−22) Fluorine is the most reactive, but because it is so reactive the products are difficult to control so it is rarely used. Chlorine reactions rates are relatively fast but not so fast that they cannot be controlled so chlorination is very common. The bromine reaction is slow and takes high levels of UV light to cause the reaction to go, but the reaction usually gives just one, or a few very selective products to it is commonly used. The reaction with iodine is so slow as to be essentially non-existent so it is not used for Free Radical Halogenation. It is important to note that Free Radical Halogenation does not occur just anywhere on a molecule. The halogens prefer to replace the hydrogen’s found on the most substituted carbons. That’s just a fancy way of saying that halogenation will occur preferentially on a tertiary carbon if one is present, or in the absence of a tertiary carbon, halogenation will occur on a secondary carbon. Actually, this reaction can be quite selective. Consider the following table, Reactivity of Halogens on Various Carbons 1° 2° 3° Chlorine 1 3.5 5 Bromine 1 97 ∞ (infinite) This table shows us that a tertiary carbon is five times more reactive than a primary carbon while being chlorinated, and infinitely more reactive when brominated. If a tertiary molecule is brominated, the bromine will end up on the tertiary carbon and nowhere else. Overall, we can see that a halogen will end up on the most substituted carbon during Free Radical Halogenation. General Chemistry | 233 The Free Radical Halogenation of an alkane is the first reaction learned by most students in organic chemistry. The reason, of course, is because it takes a relatively inert alkane hydrocarbon and makes it reactive by adding a halogen. The halogen is electronegative which means that it pulls electrons toward itself which gives the halogen a partial negative charge and makes the carbon to which it is attached, slightly positively charged. These charges provide a site for further reactions to occur. Thus, the relatively inert hydrocarbon is now ready to be turned into other more interesting compounds, like alcohols. Alcohols Alcohols are organic compounds with –OH groups. Very often they are made from alkyl halides. The reaction is simple enough, Alcohols are named by adding “ol” to the root name of the longest chain. Any molecule with an “ol” on the end is an alcohol. Cholesterol, propylene glycol (anti-freeze), estradiol (a hormone), and nonoxynol-9 (spermicide) are all alcohols. When necessary, we must name where the alcohol is found along the chain. Some common alcohols and their names are given below, General Chemistry | 234 A couple of these alcohols are worthy of discussion. One of the names for propanol is n-propanol. The “n” stands for “normal” propanol and this is always understood as meaning propanol with the OH on the end carbon. We would also write n-butanol and n-pentanol if these alcohols had their OH on their end carbons. The other alcohol that should be discussed is 2-propanol which is more commonly known as isopropyl alcohol or rubbing alcohol. The term “isopropyl” is common in organic chemistry so it is important that it be understood. You will notice that the term “isopropyl” as well as “t-butyl” have the familiar “yl” ending that indicates that it is a carbon side group, in this case, it is a side group to an alcohol OH group. All “iso” groups can be written as (CH3)2CH(CH2)n- where n can be any number between 0 and 3. The term “iso” means “the same” so isobutyl means that this compound has the same number of carbons as butane, but a different structure. General Chemistry | 235 (CH3)2CH- (CH3)2CHCH2- (CH3)2CH(CH2)2- Alcohols that are smaller than about 6 carbons are soluble in water. If the alcohol is any larger than about six carbons, the alcohol becomes insoluble. Even though an alcohol has an –OH group, it is not a base. Alcohols behave like acids, that is, they give off an H+ ion. This is because the carbon-oxygen bond tends to be very strong. The resulting structure behaves like water, where the oxygen gives off an H+ ion. Most of the reactions done with alcohols can be explained by remembering that alcohols behave as acids. Aldehydes and Ketones Aldehydes and ketones are two of a large class of compounds called carbonyls. Carbonyl compounds include aldehydes, ketones, acids, amides, acid halides and esters. What all carbonyls share in common is the functional group, C=O. What makes aldehydes and ketones different from other carbonyls is that these compounds only have a C=O and no other electron withdrawing groups. The only difference between an aldehyde and a ketone is where the C=O is found. If the C=O is found on a primary carbon, then the molecule is an aldehyde, and if it is found on a secondary compound then the molecule is a ketone. In terms of nomenclature, an aldehyde always ends in –al, and a ketone ends with an –one. Common ketones are acetone (used in fingernail polish remover) and testosterone (a male sex hormone). Most aldehydes are known by General Chemistry | 236 their common names. Formaldehyde is use in embalming and benzaldehyde is the active ingredient in almond extract. Glucose is an aldehyde even though it is not named like an aldehyde and fructose is a ketone even though there is no indication of the presence of a ketone in its name. Aldehydes and ketones are usually made by oxidizing an alcohol though many other methods could be used. General Chemistry | 237 Carboxylic Acids Carboxylic acids have the functional group, COOH. They are named by adding -oic acid to the root name of the molecule. Some carboxylic acids are shown below along with their common names. Carboxylic acids are generally soluble in water but when the carbon chain exceeds about six carbons (hexanoic acid/caproic acid) the compounds become insoluble. Very large acids like stearic acid are insoluble and they will float on the surface of water. Nearly all oils are very large acid molecules and when they get about twelve carbons long they are called “fatty” acids. Both lauric acid and stearic acid are fatty acids. All carboxylic acids are weak acids. They only partially dissociate in water. Vinegar (acetic acid) is the most common carboxylic acid, and it smells the best. Butanoic acid smells like rancid butter and hexanoic acid is the source of order among goats. The Latin name for goat is “caprinus” from which caproic acid derives its name. General Chemistry | 238 Esters Many esters smell very good. The smell of a banana, wintergreen, a crisp green apple, and pineapple are all due to the presence of esters. As a consequence, esters were once used in perfumes. Unfortunately, the chemical link that makes an ester is also easily broken and so you could leave home smelling like cherries (geranyal butyrate) but return home smelling like rancid butter (butyric acid). As a consequence, esters are no longer used in perfumes. Since esters are easily broken, pharmaceutical companies use the property to their advantage. Acids often cause indigestion while esters do not. Products like Ester-C effectively change the acidic property of vitamin c (ascorbic acid) but make the vitamin fully available since the ester bond will be broken and the vitamin made available in the gut. Aspirin is also an ester whose active ingredient is salicylic acid but whose ester is more easily tolerated. Esters are made by linking an acid with an alcohol. The link occurs through the oxygen on the alcohol rather than the acid. The water that is produced is made from an H+ coming from the alcohol and the OH- from the acid. This means that the alcohol is acting like an acid (an important point that was made earlier) and the acid is acting like a base. The general reaction is shown below, Ester Nomenclature The name of an ester begins with the alcohol and ends with the name of the acid. They alcohol takes the root name of the alcohol and adds a –yl and the acid removes the –ic acid ending and adds an –ate. In our example, the alcohol is ethanol and the acid is ethanoic acid so the name of the ester is, ethanol + yl ethanoic acid + ate ethyl ethanoate In the example above, ethanol reacts with ethanoic acid. The resulting compound is ethyl ethanoate. Some representative esters are shown below. General Chemistry | 239 Ester Name Formula Odor or occurrence Allyl hexanoate pineapple Benzyl acetate pear, strawberry, jasmine Ethyl pentanoate apple Butyl butyrate pineapple Ethyl acetate nail polish remover, model airplane glue Ethyl butyrate banana, pineapple, strawberry Ethyl hexanoate pineapple, waxy-green banana Methyl salicylate (oil of wintergreen) Modern root beer, wintergreen Amyl acetate (pentyl acetate) apple, banana Amines Esters smell very nice. Amines on the other hand do not. Consider yourself fortunate if the amine only smells as bad as ammonia. Most amines have a very offensive smell and their names often suggest the foul nature of their odor. General Chemistry | 240 Names like putrescine and cadaverine suggest the odor of rotten and dying flesh and the strong odor of fish is also associated with amines. Amines are soluble in water since they can hydrogen bond with water. Like most other organic compounds, only the smaller amines are soluble because as the chain length grows, dispersion forces take over making the compounds insoluble. Amine Nomenclature Amines are distinguished by the presence of an –NH2 on an alkane. There are two ways in which this group is used in the name of a compound, either as a side group called an “amino” group, or as the main functional group when it is called an “amine.” When an –NH2 is the main functional group “amine” is added as a suffix to the root name of the compound. Unfortunately, there is great variation in the nomenclature of amines and sometimes the carbon chain is expressed like a side group and given a –yl ending, and depending on the situation, the –NH2 can be described as a side group and it takes on the typical “o” of a side group (chloro, oxo, amino). CH3-CH2-CH2-NH2 propane + amine = propanamine propylamine aminopropane All three of these names can be found as the name of this compound. Many compounds that do not end in “amine” are amines but their name will indicate the presence of the amine by ending in “in” or “ine.” There are many such compounds that we use in daily life, most of them act as drugs. Penicillin Amoxicillin Epinephrine Ephedrine Morphine Dopamine Serotonin Acetaminophen General Chemistry | 241 Primary Amines Amines can be divided into four groups, primary, secondary, tertiary, and quaternary amines. These designations indicate the number of carbon chains attached to the amine group. Propanamine is a primary amine since it has only one carbon chain attached to it. Other primary amines are given below. Secondary Amines Secondary amines are compounds with two carbon chains attached to the amine group. Very often, the longest carbon chain is chosen as the root name and the shorter carbon chain is selected as a side group. Technically, it is possible to have the shorter side group found anywhere on the compound, but since it is attached to the nitrogen (as opposed to the longer main chain) then an “N” is used to describe this position. For example, suppose that we have the following compound; CH3-NH-CH2-CH2-CH3 The longest chain has three carbons so the root name of this compound, including the amine group, is propanamine. Since this compound also has a methyl group attached to the nitrogen, the full name of this compound would be, N-methylpropanamine. It is also possible to name this compound by listing the carbon chains as side groups. In this case, this compound would be called methylpropylamine or Nmethylpropylamine. In similar fashion, if both side chains are the same, then the name of the following compound would be diethylamine, CH3-CH2-NH-CH2-CH3 diethylamine A number of secondary amines are given below, General Chemistry | 242 Tertiary Amines Tertiary amines have had all of their hydrogen’s replaced with carbon chains. Even without a nitrogen-hydrogen bond, these molecules can still bond through their lone pair electrons so these compounds are usually soluble in water.