Carbonyl Chemistry 3: Reactions

Question 1

How do you form Grignard reagents?

Answer 1

Reaction of alkyl and aryl halides with elemental magnesium

Question 2

What is an organometallic compound and why are they useful?

Answer 2

One that contains a C-metal bond
The C-Mg bond is highly polarised due to the large electronegativity difference between the two
As a result the carbon has a greater share of the electron density making it nucleophilic

Question 3

What are the two stages to reaction with Grignard reagents?

Answer 3

1) Addition of the Grignard reagent
2) Protonation of the alkoxide intermediate by ‘acidic workup’

Question 4

Describe how Grignard reagents can act as nucleophiles with aldehydes and ketones.

Answer 4

The R group is nucleophilic due to the positive charge on the Mg
Allows R- to be attracted to the C+ on a carbonyl
Forms an intermediate alkoxide
Must then be protonated to form alcohol product

Question 5

Give two examples of hydride reagents.

Answer 5

NaBH4
LiAlH4
Both are sources of hydride ions
Exist as ion pairs
i.e. Na+ and BH4(-)
The negative charge is distributed across the whole anion
However B and Al are more electronegative than hydrogen, so the negative charge predominately resides on the hydrogen atoms, providing H- ions

Question 6

Describe how hydrides can be added to aldehydes and ketones.

Answer 6

1) Addition of LiAlH4 or NaBH4
2) Protonation of the alkoxide intermediate by acidic workup
Ketones react via the same mechanism but are less reactive

Question 7

Give examples of other nucleophiles that can be used for nucleophilic addition.

Answer 7

R’MgBr and H+
LiAlH4/NaBH4 and H+
R’Li and H+
NaCN/KCN and H+