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1 Organic Compounds – Functional Groups and Physical Properties Allotropes of Carbon – Allotropes are forms of the same element that have different bonding patterns or arrangements. Elemental carbon can exits in many different allotropes. The allotropes cover a wide range of properties and characteristics. Four allotropes are described as follows: (i) Diamond – In diamond, every carbon atom is bonded to four other carbon atoms in a tetrahedral pattern. The covalent bonds between the atoms are extremely strong, and the arrangement and symmetry of these bonds make diamond unusually strong and hard. (ii) Graphite – This soft allotrope exists in abundance. In graphite, carbon atoms are arranged in sheets or layers. These layers are held together by weak attractive forces. Graphite’s structure makes it a good writing material. The layered structure also gives it good lubricating properties as it is used to make key slide more easily into a lock. (iii) Amorphous Carbon – This allotrope has no predictable arrangement. It has random bonding pattern. Amorphous carbon is usually produced when carbon compounds decompose. Examples are: charcoal, soot, bone black (from decomposition of animal bone) and coke (from decomposition of coal). (iv) Fullerenes – Fullerenes, also called Buckyballs, are globe-shaped, cage-like arrangements of carbon atoms. A 60-carbon fullerene is found in the soot. A fullerene has been found to attack an enzyme found in the virus that causes AIDS. Unique Bonding of Carbon – Carbon’s half-filled valence level and relatively small size give it its unique bonding properties. A carbon atom needs 4 more electrons to satisfy the octet rule. Therefore carbon atoms form exactly 4 covalent bonds with other atoms. These bonds may take the forms of four single bonds, a double bond and two single bonds, or a triple bond and one single bond. A carbon atom’s electrons fill only two principal energy levels, so its valence electrons are relatively close to the nucleus. This closeness allows carbon to form short, strong covalent bonds. In addition, a carbon atom is small enough to share one, two, or three pairs of electrons with other atoms. However, carbon atoms differ from other small atoms because carbon does not typically exist as diatomic molecule. Only under exceptional circumstances can atoms share four pairs of electrons in a chemical bond. A pair of carbon atoms would need to share four pairs of electrons to form a stable C2 molecule. Long chains of carbon atoms provide the framework for an enormous variety of compounds, such as DNA, proteins, carbohydrates, and other compounds. The Nature of Carbon – Carbon Multiple Bonds: In a carbon-carbon multiple bond, either a double bond or a triple bond, there are two types of bonds present. These bonding types are known as sigma bonds or pi bonds. There are three key concepts about chemical bonding: (1) Different types of atomic orbitals are available for electrons to occupy and that these different orbitals types, s, p, d, and f, differ in shape. An “s” orbital has a spherical shape. A dumbbell shape is associated with a “p” orbital, and so on. (2) Electron sharing in covalent-bond formation results from the overlap of atomic orbitals. Because of orbital overlap, shared electrons are able to move around in the area between two covalently bonded atoms. (3) Single-, double-, and triple-covalent bonds respectively involve the simultaneous overlap of one, two, and three pairs of atomic orbitals. The manner in which two atomic orbitals overlap in the formation of a covalent bond is what determines whether a given bond is a sigma bond or a pi bond. A sigma bond is a covalent bond in which the overlap between atomic orbitals lies along the axis joining the two bonded atoms, i.e., end-to-end overlapping. A pi bond is a covalent bond in which the overlap between atomic orbitals is above and below (but not on) the inter-nuclear axis, i.e., sideby-side overlapping. (------------------------- sigma bond: three different overlap diagrams ----------------------) pi bond diagram The concepts of sigma and pi bonds relate to the various types of bonding that can occur between carbon atoms in the following ways: (i) (ii) (iii) When only one bond is present (single bond), that bond is always a sigma bond. When there is a double bond, that bond always consists of one sigma bond and one pi bond, which is formed from the overlap of one p orbital from each carbon atom. When there is a triple bond that bond always consists of one sigma bond and two pi bonds, which are formed from the overlap of two p orbitals from each carbon atom. For a carbon that has 4 single bonds, all of the orbitals are hybrids. 2 Nonmetals and Pi Bonds Carbon, oxygen, and nitrogen all have an unusual ability to form double bonds. In an atom of any of these elements, the p orbitals of the electrons available for bonding are in the second shell, not too far from the atom’s nucleus. If we drop down to the third row of the periodic table to silicon, sulfur and phosphorus, the p orbitals of the bonding electrons are farther away from the nucleus in shell three. Therefore, when third-level p electrons go into pi bonds, they cannot be as effective in holding nuclei near other as when the p electrons come from the second level. Thus, pi bonds formed by the overlap of third shell p orbitals are not as strong as those made from second shell p orbitals. Organic Compounds - Three main components of Functional Groups: (1) Multiple Bonds between C atoms −C = C − Unlike single −C − C − bonds, double and triple bonds allow atoms to be added to the chain, making −C ≡ C − these compounds more reactive, while – C – C – single bond is strong covalent bond and not reactive (2) C atoms bonded to a more electronegative atom (O, N, halogens) (3) C atom double-bonded to an O atom C=O The resulting polar bond increases boiling point and melting point. Oxidation and Reduction in Organic Reactions In general, oxidation numbers are seldom used in oxidation and reduction reactions for organic compounds. The focus is always on the increase or decrease in electron density caused by the addition or loss of hydrogen and oxygen (or other electronegative) atoms. The following guidelines are useful in deciding whether a particular organic reaction is an oxidation or reduction process. (1) A carbon atom in an organic compound is oxidized if it gains oxygen (or other electronegative) atoms or loses hydrogen atoms in a chemical reaction. (2) A carbon atom in an organic compound is reduced if it loses oxygen (or other electronegative) atoms or gains hydrogen atoms in a chemical reaction. 3 Organic Compounds Hydrocarbons (contain only carbon & hydrogen) Aliphatic Hydrocarbons Acyclic Cyclic (straight or branched structure) (has closed ring structure) Alkanes (“paraffins”) Alkenes (“ethylenes” or “olefins” ) C−C C=C Alkynes (“acetylenes”) cycloalkanes cycloalkenes Hydrocarbon Derivatives Aromatic Hydrocarbons Benzenes & its derivatives Polynuclear Aromatic hydrocarbons (“fused” benzene rings) C≡C An alkyl group is a hydrocarbon group derived from an alkane by the removal of a hydrogen atom; often a substitution group or branch on an organic molecule. An aryl group is an aromatic ring from which one hydrogen atom has been removed. Examples: CH3CH3 CH2 = CH2 Ethane Ethylene HC ≡ CH Acetylene Benzene Naphthalene Alkanes are called saturated hydrocarbons and Alkenes and Alkynes are called unsaturated hydrocarbons. Alkanes with branched carbon chains are called branched alkanes. Alkanes change their shape when a single carbon-carbon bond rotates about its axis. When two structures differ only by one or more bond rotations, they are said to be conformations of each other. Structural isomers are compounds having the same molecular formula, but their atoms bond in different orders. For example, pentane, 2-methylbutane, and 2,2-dimentylpropane are structural isomers because they all have the same formula, C5H12 . Hydrocarbons that contains a carbon ring are called cyclic hydrocarbons, e. g. the cyclic hydrocarbons with only single bonds are called cycloalkanes. Benzene is the most common and stable unsaturated cyclic hydrocarbon. The aromatic hydrocarbons are benzene and compounds containing a benzene ring. Benzene has the chemical formula C6H6 and consists of a ring of six carbon atoms. Based on the chemical formula, one proposed structure for benzene was the following: CH HC 6 1 2 CH 5 HC or CH CH 4 3 This structure would be called cyclohexatriene using the IUPAC system for naming aliphatic hydrocarbons that we have studied previously. However, the properties of benzene are very different than those of other double or triple bonded hydrocarbons. For example, benzene is a very stable molecule while alkenes and alkynes are both very reactive. In fact, benzene has 6 identical carbon-carbon bonds in its structure. Benzene can be thought of as a hybrid of two “resonance forms” of cyclohexatriene: Either of these structures is identified as benzene way to represent benzene is a ring of 6 carbons with a although neither is actually correct. Instead, a common circle in the middle: In other words, the electrons involved in the “double” bonds or pi bonds are shared equally among all 6 carbons. These pi electrons are said to be delocalized in this arrangement. Every carbon is sp2 hybridization state with one electron involved in pi bonding. This means that benzene has a planar structure as shown in these models: a) 6 half-filled p orbitals b) π-bonding (delocatized electrons) c) electron density diagram Property Characteristics of Typical Organic and Inorganic Compounds Organic Solubility in water Melting Point Boiling Point Decomposition Insoluble Low Low Occurs easily when heated Reaction with O2 Combustion (produces CO2 & H2O ) Inorganic Soluble High High Requires very high temperatures No combustion 4 Some Important Classes of Organic Compounds Class 1 Characteristic Structural Features of Molecules Hydrocarbons Prefixes in naming: Prefix No. of C’s Alkyl Grp H Y D R O G E N meth1 eth2 prop3 but4 pent5 hex6 hept7 oct8 non9 dec10 undec- 11 methyl ethyl propyl butyl pentyl hexyl heptyl octyl nonyl decyl undecyl Examples Contain only carbon and hydrogen: May have carbon chains or carbon rings. Subclasses according to presence of multiple bonds C C | | | | C C | | − C− C− 1-a. Alkanes: all single bonds Propane CH3CH2 − CH3 CH2 = CH − CH3 1-b. Alkenes: at least one double bond −C = C Propylene 1-c. Alkynes: at least one triple bond −C ≡ C− propyne 1-d. Aromatic: at least one benzene like ring system CH ≡ C − CH3 methylbenzene Note: For a corresponding prefix for a alkyl group, just add –yl: e.g. meth- becomes methyl2 Organic Halides: –X −X 3a O Alcohols: Hydroxyl group – OH R − X , Ar − X -F fluoro, -Cl chloro, -Br bromo, -I iodo, - Chloropropane nitro At least one –OH joined to a tetrahedral carbon that has 3 other single bonds (a alkyl group: CH3CH2 CH2 − Cl CH 3CH2 CH 2 − OH Propanol R, R' , R" ) R − OH : 10 : R − CH 2 − OH , 20 : R − C| H − OH , − R' R" | 30 : R − C − OH | R' 3b O Phenols (antiseptics): Hydroxyl group – OH OH bonded with a aryl (phenol) group: ∅ X − ∅ − OH , R − ∅ − OH Phenol 4 O = Carboxylic Acids: O Hydroxyl group + || Carbonyl group = - C – OH Carboxylic group: 5 O = 6 O = Acid Anhydride O 2 carbonyl group || 7 O = 8 O = 9 O − 10 O = N − O || −C − O − C − || Carbonyl group: − C − O − Aldehydes: O Terminal || Carbonyl group : - C -H Ketones: O Non-terminal || Carbonyl group: - - C Ethers: 2 alkyl group bonded to “O” An "oxy" Amides: (very weak bases) O || Carbonyl group − C− NH2 + Ammonium || || || R[ H ] − C − OH , Ar − C − OH O|| Propanoic acid CH 3 − CH 2 − C − OH O|| O|| O|| Acetic anhydride CH3 − C − O − C − CH3 R− C −O− C − R Ester (acid derivative): O O O O O O O || || ethanoate CH 3 − C − O − CH 3 || R H − C − O − R' , Ar − C − O − R O Methyl (methyl acetate) O O || || || R H − C − H , Ar − C − H Propanal O O O O || || CH3 − CH2 − C − H || || R − C − R' , R − C − Ar , Ar − C − Ar ' Propanone (acetone) CH3 − C − CH3 R − O − R' , R − O − Ar , Ar − O − Ar Methoxylethane CH3OCH2 CH3 dimethyl ether CH3OCH3 O H O H || || | | O R" || | R H − C − N − H , R H − C − N − R ' , R H − C− N − R' , O || Ar − C − NH2 Propanamide O H || | CH 3CH 2 − C − N − H 5 11 N − Amines: (weak bases) H in ammonia ( NH 3 ) replaced with alkyl groups Class 1 H H R" | | | R' | R − N − H , or R − N − R ' , or R − N − R ' ; Ar − N − R Methylamine Dimethylamine Characteristic Structural Features of Molecules Degree of Polarity Contain only carbon and hydrogen: May have carbon chains or carbon rings. Subclasses according to presence of multiple bonds 1. non-polar C C H Y D R O G E N Propylamine CH3CH2 CH2 NH2 | | | | C C | | − C− C− 1-a. Alkanes: all single bonds 1-b. Alkenes: at least one double bond −C = C 1-c. Alkynes: at least one triple bond −C ≡ C− CH3 NH2 , CH 3 NHCH 3 • Insoluble in water • Low boiling & melting points • Extreme flammable • Van der Waals force 1-d. Aromatic: at least one benzene like ring system Note: For a corresponding prefix for a alkyl group, just add –yl: e.g. meth- becomes methyl2 Organic Halides: –X −X 3a O Alcohols: Hydroxyl group – OH R − X , Ar − X -F fluoro, -Cl chloro, -Br bromo, -I iodo, - nitro At least one –OH joined to a tetrahedral carbon that has 3 other single bonds (a alkyl group: 7. polar, & more polar than alcohols 6. polar, & can form hydrogen bond R, R ' , R" ) R − OH : 10 : R − CH 2 − OH , 20 : R − C| H − OH , − R' R" | 30 : R − C − OH | R' 3b O 4 O = 5 O = 6 O = 7 O = 8 O = 9 O − Phenols (antiseptics): Hydroxyl group – OH Carboxylic Acids: O Hydroxyl group + || Carbonyl group = - C – OH Carboxylic group: O Acid Anhydride O 2 carbonyl group || || −C − O − C − OH bonded with a aryl (phenol) group: ∅ X − ∅ − OH , R − ∅ − OH || Carbonyl group: − C − O − Aldehydes: O Terminal || Carbonyl group : - C -H Ketones: O Non-terminal || Carbonyl group: - - C Ethers: 2 alkyl group bonded to “O” || || O|| 4. polar, but less polar than alcohols O|| R− C −O− C − R Ester (acid derivative): O O O R[ H ] − C − OH , Ar − C − OH O O || || R H − C − O − R' , Ar − C − O − R O O || || R H − C − H , Ar − C − H O || 5. polar, but less polar than alcohols 5. polar, but less polar than alcohols O || O || R − C − R' , R − C − Ar , Ar − C − Ar ' R − O − R' , R − O − Ar , Ar − O − Ar 3. less polar than carboxylic acids (relatively non-polar), but more polar than ethers 3. more polar than hydrocarbon & less polar than alcohols; good solvent 3. more polar than hydrocarbon & less polar than alcohols; good solvent 2. more polar than hydrocarbon; Hydrogen bond is possible 6 10 O = N − 11 N − Amides: (very weak bases) O || Carbonyl group − C− NH2 + Ammonium Amines: (weak bases) H in ammonia ( NH 3 ) replaced with alkyl groups O H O H || || | | O R" || | R H − C − N − H , R H − C − N − R ' , R H − C− N − R' , 3. less polar than amines, but comparable to esters O || Ar − C − NH2 H H R" | | | R' | R − N − H , or R − N − R ' , or R − N − R ' ; Ar − N − R 5. polar, but less polar than alcohols PhysicalProperties of Organic Compounds Additional Characteristics of Alcohols • Alcohols are extremely flammable, and should be treated with caution. • Most alcohols are poisonous. Methanol can cause blindness or death when consumed. Ethanol is consumed widely in moderate quantities, but it causes impairment and/or death when consumed in excess. Additional Characteristics of Carboxylic Acids • Carboxylic acids often have unpleasant odours. For example, butanoic acid has the odour of stale sweat. 7 • The−OH group in a carboxylic acid does not behave like the basic hydroxide ion, OH . Oxygen has a high electronegativity (attraction to electrons) and there are two oxygen atoms in the carboxylic acid functional group. These electronegative oxygen atoms help to carry the extra negative charge that is caused when a positive hydrogen atom dissociates. This is why the hydrogen atom in a carboxylic acid is able to dissociate, and the carboxylic acid behaves like an acid. − • Figure below compares the melting and boiling points of a carboxylic acid with the melting and boiling points of other organic compounds. As you can see, the melting and boiling points of the carboxylic acid are much higher than the melting and boiling points of the other compounds. This is due to the exceptionally strong hydrogen bonding between carboxylic acid molecules. Additional Characteristics of Esters • Esters often have pleasant odours and tastes, so they are used to produce perfumes and artificial flavours. In fact, the characteristic tastes and smells of many fruits come from esters. Additional Characteristics of Aldehydes and Ketones 8 • In general, aldehydes have a strong pungent smell, while ketones smell sweet. Aldehydes with higher molecular masses have a pleasant smell. For example, cinnamaldehyde gives cinnamon its spicy smell. (See Figure 1.20.) Aldehydes and ketones are often used to make perfumes. The rose ketones (shown in Figure 1.21) provide up to 90% of the characteristic rose odour. Perfumers mix organic compounds, such as the rose ketones, to obtain distinctive and attractive scents. • Since aldehydes and ketones are polar, they can act as polar solvents. Because of the non-polar hydrocarbon part of their molecules, aldehydes and ketones can also act as solvents for non-polar compounds. For example, 2-propanone (common name: acetone) is an important organic solvent in the chemical industry. • Table 1.10 compares the boiling points of an alkane, an alcohol, and an aldehyde with the same number of carbon atoms. You can see that the boiling point of an alcohol is much greater than the boiling point of an alkane or an aldehyde. Additional Characteristics of Ethers • Like alcohols, ethers are extremely flammable and should be used with caution. Additional Characteristics of Amides • An amide called acetaminophen is a main component of many painkillers. • Urea, another common example of an amide, is made from the reaction between carbon dioxide gas, CO2, and ammonia, NH3 . Urea was the first organic compound to be synthesized in a laboratory. It is found in the urine of many mammals, including humans, and it is used as a fertilizer. 9 10 Additional Characteristics of Amines • Amines are found widely in nature. They are often toxic. Many amines that are produced by plants have medicinal properties. • Amines with low molecular masses have a distinctive fishy smell. Also, many offensive odours of decay and decomposition are caused by amines. For example, cadavarine, H2NCH2CH2CH2CH2CH2NH2 contributes to the odour of decaying flesh. This compound gets its common name from the word "cadaver" meaning "dead body". • Like ammonia, amines act as weak bases. Since amines are bases, adding an acid to an amine produces a salt. This explains why vinegar and lemon juice (both acids) can be used to neutralize the fishy smell of seafood, which is caused by basic amines. 11 Summary of Organic Compounds, Their Functions and Uses Family 1-a. Alkanes 1-b. Alkenes Structures & Features Branched-chains Cyclic Structural isomers Branched-chains Cyclic Geometric isomers Vinyl carbons Polymerization 1-c. Alkynes 1-d. Aromatics, Heterocyclic, & Fused-Ring Aromatics 2. Organic halides 3-a. Alcohols 3-a. Cyclic alcohols 3-b Phenols Benzene ring: orthoMetaPara- Extremely flammable Flammable with very high temperature Pleasant odors Some toxic & carcinogenic Polyalcohols: -diol, -triol Cyclic Aromatic Thiols Toxic Very flammable Pleasant odors Heterocyclic Fused-ring 3-c. Thiols 4. Carboxylic acids Special Characteristics Extremely flammable (i) Dicarboxylic (-dioic) (ii) Tricarboxylic acid of propane Burns easily Strong, unpleasant odors Some with distinctive odors Acids with 1 or more double bonds Keto carboxylic acids (those with a carbonyl group attached) 6. Esters (organic salts) 7. Aldehydes 8. Ketones Thioesters Polymerization: ester linkages Acid anhydrides Phosphate esters Hemiacetals & acetals Hemiacetals & acetals 9. Amides (organic salts) Amide linkage (peptide bond) Heterocyclic Polymerization 10. Amines Diamines (organic bases) Heterocyclic 11. Ethers & Epoxides Cyclic Functions & Uses • • • • • • • • • • • Gas, gasoline, fuels, and oil Pheromones Petroleum as raw materials for plastics Fuel Ethene stimulates fruit ripening Beta-carotene (Vitamin A) Pheromones Insecticides Polymerization to produce plastics Welding Synthesis of alkenes and alkanes • Good solvent for numerous substances • Compounds give distinctive aromas to cloves, vanilla beans, and almonds • Compounds containing rings similar to benzene include nucleic acids (DNA), several vitamins, hormones, & pharmaceuticals • Use as intermediates in the synthesis of other organic compounds for ease of substituting a halogen • Refrigerant & aerosol propellant (CFCs being replaced by (HFCs) • Anesthetic • Gasoline additive • Freezing agent • Dry cleaning solvent (tetrachloromethane commonly known as tetrachloride now replaced by dichloromethane (methylene chloride)) • Pesticides including DDT (now banned) and chlordane • Good solvent for polar & nonpolar compounds • Rubbing alcohol • Antifreeze • Lotions, soap, shaving cream, glycerin suppositories • Flavoring, skin lotion, throat lozenges • Cholesterol • Antiseptic and disinfectant • Indicator for acid-base titration • Preparation of plastics, drugs, dyes, & weed killers • Some exert profound pyschological effects • Epinephrine & hydoquinone • Some oxidized in fruits & vegetables • Addition to gas for detection of gas leakage • Weak acids in citrus fruits, milk, yogurt • Acetic acid, first isolated from vinegar, used to produce other chemicals & to prepare foods e.g. pickles, mayonnaise, salad dressing. • Carboxylic salts as food preservatives, soaps Most pleasant odors of fruits & flowers • Flavoring in processed foods • Scents in cosmetics and perfumes Appealing tastes and fragrant odors • • • • • Appealing tastes and fragrant odors Components of proteins Unpleasant odors • • • • • • • • • • • • • • • • • Antiseptic and disinfectant Synthesis of resins and dyes, and preservatives Formaldehyde trimer to fumigate rooms against pests Acetaldehyde trimer for hypnotic drugs Used as flavorings in food and candy (such as vanilla flavoring in ice cream, and cinnamon) and as fragrance in inhalants and perfumes Good solvent for both polar & nonpolar substances Important in functional of human body Used as flavorings in food and candy and as fragrance in inhalants and perfumes Good solvent for both polar & nonpolar substances (acetone, a volatile liquid, & its vapor is quite flammable, is a common solvent, a primary component in fingernail polish remover) Important in functional of human body (sex hormones, & cortisone) Pheromones Amide group occurs in proteins & peptide bonds form backbone of all protein molecules Present in synthetic fibers such as nylon Nicotine, coniine, piperine, cocaine, caffeine Ptomaines Amino group present in DNA, vitamins, & anesthetic drug (Novocain) Basis of addictive compounds such as nicotine, cocaine, and amphetamines Epinephrine & hydoquinone Solvent for fats & oils (organic compounds) Anesthetics (in19th century) Sterilization of medical equipment A third ether, one of the methyl butyl ethers, is used in gasoline to reduce pollution and improve engine efficiency