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WEEK 7 ORGANIC CHEMISTRY NATURE OF ORGANIC CHEMISTRY Organic chemistry is that branch of chemistry that deals with certain carbon compounds. There are certain essential differences between organic and inorganic chemistry: INORGANIC CHEMISTRY 1. Molecules are relatively small 2. Molecules are relatively ionizable. 3. Reactions involve a major change in the molecule 4. Many reactions occur instantaneously 5. Four type of compounds ORGANIC CHEMISTRY 1. Molecules are relatively large 2. Molecules are relatively nonionizable 3. Reactions involve a change in only a small part of the molecule 4. Most reactions take place slowly 5. At least 12 types of compounds PROPERTIES OF CARBON The properties and reactions of organic compounds are ultimately the function of the elements of which they are made. Let us examine some of the characteristics of the element carbon. COMBINING CAPACITY – Carbon has a combining capacity of 4. Carbon has four valence electrons in its outermost energy level. When we say that carbon has a combining capacity of 4, we mean that it forms four bonds. We can represent the bonds as follows: C Carbon may combine with four univalent atoms (atoms that need one electron to fill their outermost shell). An example of a univalent atom is hydrogen: H C H H H Carbon may combine with two divalent atoms like oxygen. Oxygen needs two electrons to fill its outer shell. O C O Carbon can combine with two univalent atoms and one divalent atom: H H C O Carbon may also combine with one univalent and one trivalent atom (ex: Nitrogen): HC N THE CARBON-TO-CARBON BOND Carbon has a great tendency to combine with itself to form large molecules. Carbon may be bound to adjacent carbon atoms by one, two, or even three bonds. Carbon has a combining capacity of 4, but the maximum number of bonds between two adjacent carbon atoms is 3. C C C C C C Organic compounds form covalent bonds because the electrons between the bonds are shared. FUNCTIONAL GROUPS Combining capacities are easy to remember in organic chemistry, because they are one, two, three, and four: hydrogen, 1; oxygen, 2; nitrogen, 3; carbon, 4. The essential compounds are combinations of these elements and are organized into 12 elementary groups. These groups are identified by what are called FUNCTIONAL GROUPS. The twelve functional groups are: 1. Alkanes contain only C-H and C-C single bonds 2. Alkenes C C 3. Alkynes C C 4. Arenes C C C C C C 5. Alcohols C OH 6. Ethers C O C 7. Aldehydes O C 8. Ketones C O C 9. Carboxylic acids O C OH 10. Esters O C O 11. Amines H H N N N H 12. Amides O C NH2 O O C NH C N The functional groups are drawn with open bonds and do not represent complete compounds. The open bonds indicate where other atoms or groups of atoms attach to the functional group. The site of most organic reactions is the functional group. The rest of the molecule usually does not participate in the reaction. The functional groups alkanes, alkenes, alkynes, and arenes are collectively referred to as HYDROCARBONS because they only contain hydrogen and carbon. Another way to write the functional groups is by combining them with an “R” group or an alkyl group. We will discuss alkyl groups more later. These are called condensed structures. The condensed structures that we need to be concerned with are as follows: Alcohols Ethers Aldehydes Ketones Carboxylic acids Esters Amines ROH ROR RCHO RCOR RCOOH RCOOR RNH2, R2NH, R3N FORMULAS Three types of formulas are used in organic chemistry. 1. MOLECULAR FORMULAS – These represent the actual number of each constituent atom in the molecule. For example: The molecular formula for formaldehyde is CH2O. The molecular formula for methyl alcohol is CH4O. 2. STRUCTURAL FORMULAS – These represent the spatial relationship of the constituent atoms to each other. The structural formula for formaldehyde is: O H C H The structural formula for methyl alcohol is: OH H C H H 3. LINE FORMULAS – These are used to conserve space over structural formulas and are nothing more than condensations of them. The line formula for formaldehyde is HCHO. The line formula for methyl alcohol is CH3OH. WEEK 7 HYDROCARBONS The hydrocarbons are organic compounds containing only hydrogen and carbon as constituent elements. The physical state of the hydrocarbons depends on the length of the carbon chain. C to C4 are gases C5 to C17 are liquids C18 and higher are solids Hydrocarbons are the simplest of the organic compounds containing only two elements, hydrogen and carbon. Adjacent carbons may be joined together by one, two, or three bonds. ALKANES Hydrocarbons in which carbons are joined together by single bonds are called ALKANES. They are named according to the number of carbon atoms in the chain and an –ane suffix. The number of carbon atoms are named according to the following: 1 – Meth 2 – Eth 3 – Prop 4 – But 5 – Pent 6 – Hex 7 – Hept 8 – Oct 9 – Non 10 – Dec Thus the structural formula of methane is: H C H H H and the molecular formula is CH4. The structural formula for ethane is: H H H C C H H H and the molecular formula is C2H6. The structural formula of propane would be: H H H H C C C H H H H and the molecular formula is C3H8. Notice that carbon always has four bonds attached to it. The molecular formula is determined by counting the number of carbons and the number of hydrogens. There is a fixed ratio between carbon and hydrogen in alkanes that can be expressed by a TYPE FORMULA: CnH2 (n)+2 (n is equal to the number of carbons). If we know the number of carbons, we can determine the number of hydrogens. For example, if we have 5 carbons the number of hydrogens is 2(5) +2 = 12. This alkane would be pentane because it has 5 carbons and the molecular formula would be C5H12. Alkanes are also called SATURATED HYDROCARBONS because single bonds between adjacent carbon atoms allow attachment of the maximum number of hydrogen atoms. Remember a saturated solution contains the maximum number of solute the solvent can hold. ALKYL GROUPS Alkanes are represented by the type formula CnH2n+2. If a hydrogen atom is removed from an alkane, the type formula becomes CnH2n+1. The new structure is no longer a complete compound but now is a group of atoms capable of attaching to another atom at the place where the hydrogen was removed. This structure is called an ALKYL GROUP. They are named by replacing the –ane ending of the parent hydrocarbon with –yl. Methane Ethane Propane Butane Pentane CH4 C2H6 C3H8 C4H10 C5H12 becomes becomes becomes becomes becomes methyl ethyl propyl butyl pentyl Structurally an alkyl group would look like the following: H H C H Methyl H H H H C C C H H H Propyl CH3 C2H5 C3H7 C4H9 C5H11 Notice the missing hydrogen in both compounds. ALKYL HALIDES Alkyl groups may be attached to Group 7A (halogen) elements. The substances that are formed are called ALKYL HALIDES. Alkyl halides have the general formula RX Alkyl group Halogen In this formula, “R” is from the German word Radikal, which means, “group”. The simplest alkyl halide occurs if we replace one hydrogen on a methane with a chlorine: H C Cl H H methyl chloride (chloromethane) We can continue to obtain more alkyl halides by replacing additional hydrogens on the methane. Two of the more common compounds are: H Cl C Cl Cl C Cl Cl Cl Cl Chloroform Carbon tetrachloride ISOMERS Alkanes tend to form ISOMERS. Two or more substances having the same molecular formula but different structural formulas are Isomers. Butane is the first member of the alkanes to form an isomer. H H H H H C C C C H H H H H H H H H C C C H H H H C H Normal butane (n-butane) H Isobutane The molecular formula for both normal and isobutane is C4H10. However, their structural formulas show that there is a difference between the two compounds. The line formulas also show a difference. The line formula for Normal butane is CH3CH2CH2CH3 and the line formula for isobutane is CH3CHCH3CH3. ALKENES When multiple bonds are present in a carbon chain, the substance is referred to as an UNSATURATED HYDROCARBON. It is called this because it contains less hydrogen atoms than it could if only single bonds were present. The simplest unsaturated hydrocarbons contain one double bond in the chain. Alkenes are hydrocarbons containing only double bonds. The simplest alkene is ethene: H2C CH2 The eth- indicates the two carbons and the –ene indicates the double bond. When naming alkenes we use the same prefixes learned for alkanes (meth, eth, prop, but...) but we use the –ene ending to let us know we have a compound with a double bond. As we did for alkanes and the alkyl group, we can write a type formula for alkenes. The formula is CnH2n. ALKYNES Another form of unsaturated hydrocarbons contains a triple bond between adjacent carbon atoms. The first member of this series is ethyne or acetylene: H C C H The type formula for the Alkynes is CnH2n-2. Alkynes are named using the prefixes we learned (meth, eth, prop...) followed by the suffix –yne. Alkynes contain only one triple bond. If we have two triple bonds we then have a diyne and three triple bonds would be a triyne. CYCLIC HYDROCARBONS So far the hydrocarbons we have looked at have all been open-chain compounds. Hydrocarbons also exist as ring or cyclic compounds. They are named by adding the prefix cyclo- to the name of the corresponding open chain compound. The type formula for the cycloalkanes is CnH2n and for cycloalkenes is CnH2n-2. Cyclopropane is an example of a cyclic hydrocarbon. H2 C CH2 C H2 AROMATIC HYDROCARBONS A major way to classify all organic compounds is as ALIPHATIC or AROMATIC. The word aliphatic means “fatty”. The alkanes, alkenes, alkynes, and their cyclic derivatives are aliphatic compounds. The connection between the word “fatty” and these compounds is that fats contain a hydrocarbon portion that often resembles long-chain alkanes or alkenes. Compounds classifies as aromatic contain a benzene ring or a system of these rings. These compounds, also known as ARENES, are called aromatic because the first ones discovered had pleasant odors. Examples are vanilla and oil of wintergreen. STRUCTURE OF BENZENE The molecular formula of benzene is C6H6. It is a ring compound. In order for each carbon to have four bonds, chemists who first studied benzene described it as a ring compound with alternating double and single bonds. This cannot be true since all the bond lengths in a benzene ring are equal. Carbon- to- carbon double bonds are shorter than carbon-to-carbon single bonds. If benzene did have alternating double and single bonds, then it would undergo typical alkene reactions. This is not the case. Benzene consists of six equal carbon-to-carbon bonds that are intermediate in properties between a double and single bond. The structure of benzene is represented as follows: The arrow indicates that the molecule’s true structure is a blend of what are drawn as two contributing structures. Each of the corners of the hexagons represents a carbon atom bonded to a hydrogen. The only difference between the two structures is the placement of the double bonds. Whenever two or more structures that differ only in the position of electrons can be drawn for a compound, that compound has the property of RESONANCE. This is characteristic of all aromatic compounds. Another way which we can represent benzene is: This is now the more accepted structure of benzene. This structure represents the fact that the electrons in the double bonds are free to move about the benzene molecule and are not just “stuck” between the carbon atoms forming the double bond. This explains why the bond lengths are all the same in benzene. The circle represents electron delocalization (the electrons are not “stuck” in one place) and is a stabilizing factor for benzene. The molecule is unreactive. The major reactions of benzene generally occur only under strenuous conditions and involve substituting other atoms for the hydrogens. Benzene is a good organic solvent and the starting material for the synthesis of many widely used compounds. DERIVATIVES OF BENZENE We will focus on only the monosubstituted (only one hydrogen replaced) and disubstituted (two hydrogens are replaced) benzenes. Some monosubstituted benzenes are known by common names. Important compounds in this classification are Toluene (methylbenzene), phenol (hydroxybenzene), and aniline (aminobenzene). Toluene is an extremely good solvent for paints and varnishes. It is the starting material for trinitrotoluene (TNT). Their structures are as follows: OH NH2 Phenol Aniline CH3 Toluene Because all six carbon-to-hydrogen bonds are equivalent in benzene, only one form of each of the monosubstituted benzenes exists. The story is different for the disubstituted benzenes. Three isomers of each exist. Consider the compound dichlorobenzene. There are three possible ways to draw its structure. Cl Cl Cl Cl Cl ortho-dichlorobenzene meta-dichlorobenzene Cl para-dichlorobenzene Each of these represents an individual compound with different properties. To distinguish among them, chemists use a nomenclature with the prefixes ortho, meta, para. If two substituents on a benzene ring are on adjacent carbons, the compound is the ORTHO isomer. When one carbon separates the positions of the two substituents, the molecule is the META form. Finally, if the substituents are directly across the ring from each other the compound is the PARA isomer.