Ch. 18 Aldehydes an Ketones- Nucleophilic Addition (Part 2)

during the formation of a cyclic hemiacetal, the hemiacetal carbon (previously the carbonyl carbon) becomes a new ____ center. This leads to a pair of stereoisomer products

chiral

Hemiacetals are usually not stable, but hemiacetals containing a ______ or _______ membered cyclic structure are stable molecules

5 6

Describe the mechanism of acetal formation from an aldehyde or ketone

A hemiacteal is formed as an intermediate Te first step of the reaction is protonation of the carbonyl O atom The alcohol acts as a nucleophile in the reaction

_______ are molecules that are commonly referred to as sugars and starches and contain multiple hydroxyl group

Carbohydrates

When a monosaccharide such as glucose forms a 6-membered cyclic hemiacetal, the OH group at ______ in the molecule acts as the nucleophile in the cyclization reaction

Carbon 5

Match each type of substituent correctly to its effect on the behavior of a carbonyl group. Assume in all cases that the substituent is directly attached to the carbonyl carbon atom EDG WEG There will be more hydrate at equilibrium There will be less hydrate at equilibrium

EDG- stabilize a carbonyl group, decreasing its reactivity WEG- destabilize a carbonyl group, increasing its reactivity There will be more hydrate at equilibrium- if there are EWG attached to the carbonyl group There will be less hydrate at equilibrium- if there are EDG attached to the carbonyl group

Aldehydes/ketones form hydrates when reacted withH2O. Match the steps of the reaction to the correct description in each case Nucleophilic attack occurs as the first step Protonation of the carbonyl oxygen is the first step Under acidic conditions Under basic conditions

Nucleophilic attack occurs as the first step- under basic conditions Protonation of the carbonyl oxygen is the first step- under acidic conditions Under acidic conditions- the electrophilic character of the carbonyl C atom is increased, increasing the rate of the reaction Under basic conditions- the nucleophile is stronger, increasing the rate of the reaction

Carbohydrates are molecules that contain multiple _______ groups and one _______ functional group

OH carbonyl

Select all the reagents and/or reaction conditions toward which acetals are stable

Oxidizing agents Reducing reagents Aqueous base Nucleophilic attack

Hydrates are formed when aldehydes/ketones are reacted with H2O. Under acidic conditions, the first step of the mechanism is _______. Under basic conditions, the first step of the mechanism is ________

Protonation of the carbonyl Addition of the nucleophile to the carbonyl

The steps involved in the mechanism of acetal formation are given. Place these in the correct order, starting with the first step at the top of the list

Protonation of the carbonyl O atom Nucleophilic attack by the alcohol Deprotonation of the conjugate acid of the hemiacetal Protonation and elimination of H2O Second nucleophilic attack Deprotonation of the conjugate acid of the acetal

Which of the following methods are used to derive acetal formation in the forward direction? (i.e. to shift the equilibrium in favor of the products)?

Removing H2O by distillation as it forms Adding a drying agent to the reaction mixture

Which of the following options correctly describes the effect of pH on the rate of hydrate formation from an aldehyde or ketone?

The reaction rate increases under acidic conditions Under acidic conditions, the first step of the reaction is the protonation of the carbonyl oxygen atom The reaction proceeds most slowly under neutral conditions

When a diol (HO(CH2)nOH) is reacted with a ketone or aldehyde under acidic conditions, a cyclic ________ is formed

acetal

Under neutral conditions, hydrate formation is slow. The reaction of water with an aldehyde or ketone is best performed using a(n) ______- catalyst

acid or base

A cyclic hemiacetal can be converted to an acetal under acidic conditions via reaction with _______

an alcohol

An acetal can be hydrolyzed to an aldehyde or ketone using ______

aqueous acid

Reaction of a carbonyl compound with H2O results in the addition of H and OH across the C=O pi bond, resulting in a product that has 2 OH groups on the former C=O carbon. This structure is called a(n) _______

gem-diol hydrate

Carbohydrates such as glucose, which contains both an aldehyde group and several hydroxy groups, can undergo an intramolecular cyclization to form a cyclic ______

hemiacetal

The species formed when an aldehyde or ketone reacts with one equivalent of alcohol is a(n) _______

hemiacetal

In a compound that contains both an aldehyde and a hydoxyl group, the ____ group can attack the ____ to form a cyclic hemiacetal

hydroxyl carbonyl C

Carbohydrates contain both a hydroxyl group and an aldehyde group that can undergo an ________ cyclization to form a hemiacetal

intramolecular

Acetals are readily formed and hydrolyzed, as well as being inert to many reactions that aldehydes and ketones normally undergo. An acetal is therefore an ideal ______ group for an aldehyde/ketone that is present with other functional groups

protecting

An acetal is produced by reaction of an aldehyde or ketone with _____ equivalents of a(n) _______

two alcohol