MCAT Alcohols

Question 1

What is the structure of alcohols?

Answer 1

Hydroxyl groups attached to an R group.

Has a set of two lone pairs on the oxygen making it have a tetrahedral shape (109.5 degrees).

Question 2

Phenols

Answer 2

Alcohols in which the hydroxyl group is attached to a benzene ring.

Question 3

How do we classify alcohols?

Answer 3

By the number of R groups attached.
- Methyl - 0 alkyl groups
-Primary - 1 alkyl group
- Secondary- 2 alkyl groups
- Tertiary - 3 alkyl groups

Question 4

Describe B.P. and M.P of alcohols?

Answer 4

Alcohols has higher M.P. and B.P. due the hydroxyl groups having both hydrogen acceptors and donors.

• Branching decreases B.P. and M.P. due to decreased surface area.
• More -OH groups = higher B.P. and M.P.

Question 5

Describe the solubility alcohols

Answer 5

Fewer than 3 carbons and alcohols has good solubility. More than 4 carbons and it has lower solubility.

Branched alcohols has higher solubility due to a decrease in surface area.

Question 6

How can we create alcohols via reduction?

Answer 6

We can use LiAlH4 or NaBH4 to create alcohols from aldehydes, ketones. Carboxylic acid derivatives reduced by LiAlH4 because it’s stronger, followed by an acidic workup.

Ketones reduced by either reducing agent create secondary alcohols.

Tertiary alcohols cannot be formed from reduction.

Question 7

Strong v. mild oxidation reagents

Answer 7

Strong oxidizing agents - facilitate double oxidation.

Mild oxidation agents - facilitate single oxidation.

Question 8

Oxidation of primary v. secondary alcohols

Answer 8

Primary alcohols oxidized to aldehydes using mild oxidation reagents. Using strong oxidizing agents create carboxylic acids.

Secondary alcohols will be oxidized to a ketone with any oxidizing agent.

You cannot oxidize a tertiary alcohol

Question 9

Describe the formation of hydroxyquinones

Answer 9

Quinone is a cyclic structure with two carbonyl carbons in the para position.

Reduction results in a semiquinone in which one carbonyl has a radical electron and the other is an alcohol group.

p- hydroxyquinones formed when the two carbonyls become two hydroxyl groups.

Question 10

What is the conjugate base of an alcohol?

Answer 10

Alkoxide ion which has a negative charge on oxygen.

EDG- destabilizes them.
EWG- stabilizes them.

alkyl groups decrease alcohols acidity v. halogens increases acidity.

Conjugate base is the phenol which is called the phenoxide ion and is more acidic due to resonance stabilization.

Question 11

Describe alcohols v. alkoxide as a nucleophile group

Answer 11

Alcohols are weak nucleophiles and so participate in SN1 reactions.

Alkoxides are strong nucleophiles and so participate in SN2 reactions.

For both sterically unhindered nucleophiles are the best.

Question 12

Fisher esterfication

Answer 12

A way to make an ester via an alcohol and a carboxylic acid under acidic conditions.

Condensation reaction in which water is released.

Question 13

Describe the process of making alcohols into a sulfonate ester

Answer 13

Happens when we react a sulfonyl chloride with an alcohol.

Two common sulfonyl chlorides are called tosyl chloride ( TsCl) and mesyl chloride ( MsCl).

Tosyl chlorides + alcohols = makes tosylate ester.

Mesyl chlorides + alcohols = makes mesylate ester.

• Mechanism starts with oxygen of hydroxyl group attacking the sulfur of the sulfonyl chloride displacing the pi bond, reforming an displacing the LG. Base deprotonates the alcohol forming sulfonic ester.
• R groups on alcohol determines which mechanism is used:
  + methyl, primary, secondary = SN2 reactions.

+ Tertiary = SN1 reactions.

• When treated with a base and heat you get either an E1 or E2 reaction.

Question 14

Describe the mechanism in making of a tosylate or mesylate?

Answer 14

The oxygen of the alcohol attacks the sulfur of the p- toluenesulfonyl chloride or p- methylenesulonyl chloride.

This causing push of electrons on oxygen, pi bond reforms displacing the chloride.

We then add a bas to get rid of the hydrogen on the alcohol. This creates the tosylate (- SO3C6H4CH3) or mesylate (- SO3CH3).

RETENSION OF STEREOCHEMISTRY BECUASE THERE IS NO ATTACK ON CARBON

Question 15

How can we convert an alcohol to an alkane?

Answer 15

We first transform the alcohol to either a tosylate ester or methylate ester. We then treat it with LiAlH4 which replaces the group with a hydrogen.

Cannot be used on tertiary sulfonate esters as the reaction is an SN2 reaction.

Question 16

How can we make alcohols into good nucleophiles by using hydrohalic acids?

Answer 16

The alcohol is reacted with a hydrohalic acid by which the oxygen gets protonated by the acid becoming an oxonium ion (a good LG). The nucleophile (the halide) then attacks displacing it.

Question 17

Lucas’s test

Answer 17

Used to determine if there’s a presence of an alcohol group.

We react solution with ZnCl2 + HCl and if it turns from clear to cloudy alcohol groups are present. We can tell what type of alcohol is present by the time the solution turns cloudy.

• Less than one minute - tertiary alcohol via SN1 mechanism.
• 1-5 minutes - secondary alcohol and is an SN1 mechanism.
• More than 5 minutes -primary alcohol via SN2.

Question 18

Describe the conversion of an alcohol to an alkyl bromide?

Answer 18

Lone pairs on oxygen attacks phosphorus on PBr3 displacing the bromide and forming an oxonium ion.

The bromide attacks, displacing the oxonium ion. SN2 reaction so the result is a walden inversion.

Question 19

How can we make conditions favored for elimination reactions?

Answer 19

Adding a base and heat. Depending on the strength of the base and steric hinderance on substrate will determine whether it’s E1 or E2.

Question 20

How can we protect alcoholic groups?

Answer 20

Through silyl ethers. Which is alkyl or aryl groups attached to a silicon group.

Silyl ethers are added in basic conditions and reversed on acidic conditions.

Reversing the protecting group is done opposite of how it was protected, this is called orthogonal activity.

Common silyl ether is TBDMS which occurs through an SN2 reaction mechanism.