Answers and Explanations - Alcohols - MCAT Organic Chemistry Review

MCAT Organic Chemistry Review


Answers and Explanations

1. BAlcohols have higher boiling points than their analogous hydrocarbons as a result of their polarized O–H bonds, in which oxygen is partially negative and hydrogen is partially positive. This enables the oxygen atoms of other alcohol molecules to be attracted to the hydrogen, forming a hydrogen bond. Heat is required to break these hydrogen bonds, thereby increasing the boiling point. The analogous hydrocarbons do not form hydrogen bonds and, therefore, vaporize at lower temperatures. Choice (A) is irrelevant; oxygen”s bond length is not a factor in determining a substance”s boiling point. Choices (C) and (D) are true statements, but are also irrelevant to boiling point determination.

2. ATertiary alcohols can be oxidized but only under extreme conditions because their carbon atoms do not have spare hydrogens to give up. Alcohol oxidation involves the removal of such a hydrogen so that carbon can instead make another bond to oxygen. If no hydrogen is present, a carbon–carbon bond must be cleaved, which requires a great deal of energy and will, therefore, occur only under extreme conditions. Choice (B) is incorrect because alcohols are not carbonyl-containing compounds. Choice (C) is incorrect because the hydroxyl group of the tertiary carbon is still polarized. Choice (D) is a false statement; tertiary alcohols are still involved in other reactions, such as an SN1 reaction.

3. BRemember, diols are named after the parent alkane, with the position of the alcohols indicated, and ending in the suffix –diol. Here the carbon chain is three carbons, with a hydroxyl group on carbons 1 and 2. Thus, the name is propane-1,2-diol.

4. BThis molecule is a phenol, not a hexanol, because the cyclic group has aromatic double bonds rather than single bonds. The methyl group is separated from the hydroxyl carbon by one carbon in between, making this molecule m-methylphenol.

5. CAll else being equal, boiling points increase with increasing size of the alkyl chain because of increased van der Waals attractions. Isobutyl alcohol has the largest alkyl chain and will thus have the highest boiling point; methanol has the smallest chain and will thus have the lowest boiling point.

6. BPhenols have significantly more acidic hydroxyl hydrogens than other alcohols, so this will be the most acidic hydroxyl hydrogen. The acidity of hexanol and cyclohexanol are close, but the hydroxyl hydrogen of hexanol is slightly more acidic because the ring structure of cyclohexanol is slightly electron-donating, which makes its hydroxyl hydrogen slightly less acidic.

7. BCH3CH2CH2OH is 1-propanol, a primary alcohol. The desired end product, CH3CH2CHO, is propanal, an aldehyde. The only reactant capable of oxidizing primary alcohols to aldehydes is pyridinium chlorochromate (PCC). Chromic trioxide and dichromate salts will both oxidize primary alcohols to carboxylic acids.

8. DCyclohexanol is a secondary alcohol, so any of the oxidizing agents listed will convert it to a ketone.

9. AAcidic conditions, provided by dilute sulfuric acid, are required to complete the Jones oxidation. This reaction is carried out in aqueous conditions, eliminating choice (C). While heat may speed up the reaction, high temperatures are not required for this reaction, eliminating choice (D).

10.AMethylsulfonyl chloride serves as a protecting group for alcohols, which are converted into mesylates. Reacting with this reagent before continuing with what would normally be an oxidation reaction keeps the alcohol from reacting; when the protecting group is then removed using strong acid, the resultant product is the same as the initial reactant. Neither of the oxidation products in choices (B) or (C), nor the reduction product in choice (D), will be formed.

11.AThis reaction will create a ketal. This is the first step of the protection of aldehydes or ketones using dialcohols.

12.DThis is the Jones oxidation, which will convert a primary alcohol into a carboxylic acid. Choice (A) is the original reacting molecule, and because the reaction will proceed, this is not correct. Because the reaction uses strong oxidizing agents, it won”t stop at the aldehyde, choice (B). This reaction also cannot make the ketone in choice (C) because it starts with a primary alcohol.

13.BIn order to convert phenols into hydroxyquinones, they must first be converted to quinones through an oxidation step; a second oxidation step is required in order to further oxidize quinones to hydroxyquinones.

14.CThe reaction that converts ubiquinone into ubiquinol is a reduction reaction in which two ketones are reduced to two hydroxyl groups.

15.AAn acetal can be converted to a carbonyl and a dialcohol by treatment with aqueous acid. This is the final step when using alcohols as protecting groups, called deprotection.