carbonyl hydrates

MS 919 - Nucleophilic Acyl Substitution and Addition Submitted by Matt on August 6, 2011.

Textbook and Chapters: Carey and Giuliano 8th Ed. (2010), Chapters 17, 18, 19

Keywords: carbonyl hydrates, nucleophilic acyl addition, nucleophilic acyl substitution

Description: The most important mechanism in second semester organic chemistry. Goes over carbonyl addition/substitution mechanisms under both acidic and basic conditions.

Total Problems: 6

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Problem # 706

Carbonyls are in equilibrium with their hydrate forms. This equilibrium happens in both acid and base.

Let's go through this equilibrium under acidic conditions. Draw a mechanism using curved arrows for each reaction below.

Remember that under acidic conditions, most species are either neutral or positively charged, and rarely negatively charged. So your structures will contain either ROH or ROH₂⁺, but not RO^-.

a) Carbonyl to Hydrate (acidic)

b) Hydrate to Carbonyl (acidic)

Solution

The interconversion between a carbonyl (sp² carbon) and a tetrahedral intermediate (sp³ carbon) is the most common mechanism you will encounter in second semester organic chemistry.

You should be familiar drawing it under both acidic (this problem) and basic (problem 705) conditions.

In a), the carbonyl "goes up" to form a tetrahedral intermediate.

In b), an oxygen "comes back down" to reform the carbonyl and kick off a leaving group.

You will see this "up, down, kick" pattern in most mechanisms that involve attack at the carbonyl carbon, which is most of the reactions in second semester orgo!

a) Carbonyl to Hydrate (acidic) "UP"

Because this reaction takes place under acidic conditions, the carbonyl must protonate before the nucleophile attacks, to prevent oxygen from ever being negative (ROH instead of RO^-).

b) Hydrate to Carbonyl (acidic) "DOWN"

The leaving group must protonate before it leaves, so it doesn't leave as a negative molecule (H₂O instead of HO^-).

Problem Details

MendelSet practice problem # 706 submitted by Matt on July 22, 2011.

Subject: Organic Chemistry

Subject Topic(s): Nucleophilic Acyl Addition (Aldehydes & Ketones, Imines/Enamines, Acetals/Ketals)

Question Type: Mechanism (show a mechanism using curved arrows..)

Keyword(s): nucleophilic acyl addition, up-down-kick carbonyl mechanism, carbonyl hydrates

Problem # 705

Carbonyls are in equilibrium with their hydrate forms. This equilibrium happens in both acid and base.

Let's go through this equilibrium under basic conditions. Draw a mechanism using curved arrows for each reaction below.

Remember that under basic conditions, most species are either neutral or negatively charged, and rarely positively charged. So your structures will contain either ROH or RO^-, but not ROH₂⁺.

a) Carbonyl to Hydrate

Notice that no oxygen is ever positive during these basic mechanisms (always negative or neutral).

b) Hydrate to Carbonyl

Solution

The mechanisms in this problem and problem 706 are the most common mechanisms you will draw during second semester organic chemistry, and so it's a good idea to draw them out a few times.

In a), the nucleophile attacks the carbonyl carbon, and the double bond goes "up" to form a tetrahedral (sp³) carbon.

In b), one of the oxygen atoms acts as leaving group, and a lone pair on the other oxygen comes "down" to reform the double bond (and the sp² carbon).

You will see this "up, down, kick" pattern in most mechanisms that involve attack at the carbonyl carbon, which is most of the reactions in second semester orgo!

a) Carbonyl to Hydrate (basic conditions) "UP"