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Chapter 5

Chapter 5 Carbonyls and Alcohols.pdf

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CHM 2210

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Chapter 5 Carbonyls and Alcohols Tuesday, July 16, 2013 9:13 PM 1. Alcohols a. Compounds that possess a hydroxyl group (OH) are characterized by names ending in "ol" b. Phenol is a special kind of alcohol c. It is comprised of hydroxy group attached directly to a phenyl ring d. Can be designated as primary, secondary, or tertiary i. 2. Nomenclatureof alcohols a. Identify and name the parent b. Identify and name the substituents c. Assign a locant to each substituent d. Assemble the substituent alphabetically e. f. 3. Physical properties of alcohols a. Quite different from the physical properties of alkanes or alkyl halides b. The boiling points are much higher due to hydrogen bonding that can occur between molecules i. c. Not all alcohols are miscible (which means any proportion can be dissolved) with water i. It depends on the length of the hydrophobic region present ii. 4. Acidity of alcohols and phenols a. As we know, the acidity of a compound can be qualitativelyevaluated by analyzing the stability of its conjugate base i. b. The conjugate base of an alcohol is called an alkoxide ion, and has a negative charge on its oxygen atom i. A negative charge on an oxygen atom is more stable than a negative charge on a carbon or nitrogen due to electronegativity ii. c. Therefore,alcohols are more acidic than amines and alkanes by less acidic than hydrogen halides i. 5. Reagents for deprotonating an alcohol a. Two commonways to deprotonate an alcohol i. A strong base can be used - 1) Sodium hydride is often used because hydride (H ) deprotonatesthe alcohol to generate hydrogen gas, which bubbles out of the solution 2) ii. The metals Li, Na or K are often used 1) These metals react with the alcohol to liberate hydrogen gas, producing the alkoxide ion 2) 6. Factors affecting the acidity of alcohols and phenols a. Resonance i. One of the most significant factors affecting acidity of alcohol is resonance b. Induction i. The conjugate base is stabilized by the electron-withdrawing effects of the nearby chlorine atoms c. Solvation effects i. The more branching, the less acidic ii. Due to sterics iii. The less branching, the more easily the alkoxide ion can be solvated and stabilized 1) 7. Alcohols are nucleophilic reagents a. They are not good nucleophiles in their protonated state, but they can be deprotonatedand convertedinto their anion b. Because alkoxides are strong bases (even though they are still slightly acidic), they are not ideal nucleophiles, but still better than alcohol 8. Preparation of alcohols via acid catalyzed hydration 8. Preparation of alcohols via acid catalyzed hydration a. Hydration of an alkene follows Markovnikov'srule b. Takes place when water is added to an alkene in the presence of a COLD ACID c. 9. Preparation of alcohols via oxymercuration/demercuration a. Markovnikovaddition of water across an alkene without carbocation rearrangements b. c. d. If an alcohol is used instead of water, the corresponding ether is produced 10. Preparation of alcohol via hydroboration a. Places the OH group on the LESS SUBSTITUTED CARBON (ANTI-MARKOVNIKOV) b. 11. Preparation of Alcohols Via substitution a. Alcohols can be prepared by substitution reactions in which the leaving group is replaced by a hydroxyl group i. Primary requires a strong nucleophile while a tertiary will require a weak nucleophile ii. 12. Aldehyde and Ketone Properties a. Responsible for many flavors and odors b. Because aldehydes and ketones do not form hydrogen bonds, they typicallyhave boiling points only slightlyhigher than alkanes of equal mass c. Aldehydes and ketonesare polar aprotic 13. Aldehyde and Ketone Spectroscopy a. Aldehydes have a stretch around 1700 and one around 2800 i. b. Ketones have one stretch around 1720 14. Ketals, Hemiketals, Acetals, and Hemiacetals 14. Ketals, Hemiketals, Acetals, and Hemiacetals a. Formed by reacting aldehydes and ketones with alcohols under different conditions b. Both Ketals and Acetals are unreactive towards bases c. Ketals and hemiketals are derivatives of ketoneswhile acetals and hemiacetalsare derivativesof aldehydes d. Ketals occur when a ketone losses the carbonyl group and gains to alkyl function groups (R- O) i. e. The oxidation state of carbon does not change because the carbon still has two bonds to oxygen, but now it has two sigma bonds to two different oxygen atoms f. A hemiketaloccurs when the ketone has its carbonyl group converted into a hydroxyl group and gains one alkyl group i. g. Acetals are similar to ketals except it is the aldehyde that loses its carbonyl group to gain the two alkoxy group i. h. Acetals and ketals are useful as protecting groups in organic synthesis i. It is a reversible process that can be controlled by carefully choosing reagents and conditions i. Acetal formationis favored by removalof water, so to convert an acetal back into the corresponding aldehyde or ketone, it is simply treated with water in the presence of an acid catalyst j. ACETALS AND KETALS CAN BE FORMED AND REMOVED ONLY UNDER ACIDICCONDITIONS (ALL INTERMEDIATSCARRY A POSITIVE CHARGE,NEVER NEGATIVE!) i. An aldehyde or ketone will react with two moleculesof alcohol to form an acetal ii. Uses an acid catalyst. 1) Specifically, in the presence of an acid, the carbonyl group is protonated, rendering the carbon atom even more electrophilic iii. iv. Notice how two equivalents of alcohol are used v. Instead, a compound containing two OH groups can be used, forming a cyclic acetal 1) k. HEMIACETALS AND HEMIKETALS ARE FORMED ONLY UNDER BASIC CONDITONS i. Hemiacetalsare very hard to isolate ii. However,when a compound contains both a carbonyl group and a hydroxyl group, the resulting cyclic hemiacetal can often be isolated iii. iv. An excellent example of a hemiacetal is glucose 1) 15. Carboxylic acids and their derivatives a. Much of the chemistry of the carboxylic acids and acid derivativescenters around changing the group on the carbonyl carbon b. Monocarboxylicacids, compounds that have only one carboxylic acid, are named with the suffix "oic" acid c. Carboxylic acids can form two hydrogen bonding interactions, which explains the relatively high boiling points i. d. When a carboxylic acid is treated with a base, it yields a carboxylate salt d. When a carboxylic acid is treated with a base, it yields a carboxylate salt i. e. Carboxylate salts are ionic and therefore more soluble in water i. They are named by replacing the suffix "ic" acid with "ate" ii. 16. Spectroscopyof Carboxylic Acids a. The most obvious feature in the infrared spectrum of a CA is the intense carbonyl stretching around 1700 and the OH stretching around 2500-3500 i. b. 17. Carboxylic Acid Derivatives a. Carboxylic acids can be reduced with LAH to a primary alcohol i. b. Carboxylic acids can also undergo reactions that do not involve a change in oxidation state i. c. Replacementof the OH group with a different group (Z) does not involvechange in oxidation state is Z is a heteroatom(CL, O, N…) d. e. Just because a carbonyl group is present doesn’t mean it’s a carbolic acid derivative f. Any compound with a carbon atom that has three bonds to heteroatoms (not carbon or hydrogen) is a carboxylic acid derivatives i. g. Remember,Carboxylic acids and carboxylic acid derivativescan be converted back and forth g. Remember,Carboxylic acids and carboxylic acid derivativescan be converted back and forth without changing oxidation state, so it is neither an oxidation or reduction 18. Acid halides a. Naming i. Named as the derivative of carboxylic acids by replacing the suffix "ic acid" with "yl [halide]" ii. b. Reactivity i. Most reactive of carboxylic acid derivatives ii. An acid halide has three resonance structures 1) iii. The third resonance structure is not very effective, and as a result, the chlorine atom does not donate much electron density to the carbonyl via resonance iv. The net effect of the chlorine atom is to withdraw electron density, rendering the carbonyl grou
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