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Nucleophilic Addition Reaction is observed in Aldehydes and Ketones. In these reactions, generally, a nucleophile forms a sigma bond with an electron of other species. These reactions help in the formation of several new organic chemicals.
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Keyterms: Aldehydes, Ketones, Electron, Nucleophile, Sigma Bond, Pi Bond, Alkenes, Polar Addition, Electrophile
Addition Reaction: Definition
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An Addition Reaction is a chemical reaction in which two or more reactants come together and form a larger single product. But it is only possible for the chemical compounds with multiple bond characters to undergo an addition reaction because a double bond or a triple bond is needed to be broken to form single bonds.
There are two variants of addition reaction for polar addition:
- Electrophilic Addition Reactions – the type of reactions observed in alkenes.
- Nucleophilic Addition Reactions – the type of reaction observed in Ketones & Aldehydes
For the non-polar addition reactions, the categories are:
- Cycloadditions Reactions
- Free radical addition reaction
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What is Nucleophilic Addition Reaction?
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Nucleophilic Addition Reaction is a type of Addition Reaction in which a nucleophile reacts with a Pi- bond of a compound and results in the formation of a new sigma bond. Nucleophilic Addition reactions are the most important reactions as it allows carbonyl compounds to form various new products with different functional groups.
So, the reaction of the nucleophile with the carbonyl group can be elaborated in three steps:
- The nucleophile forms a sigma bond with the electrophilic carbon
- The C = O pi bond breaks down which results in the formation of an alkoxide intermediate (the pair of electrons gets transferred to the oxygen atom)
- The alcohol derivative is yielded due to the protonation of the alkoxide
Carbonyl Compounds and Nucleophilic Addition Reaction
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The carbonyl compounds usually undergo nucleophilic addition reactions due to the polarity of the C = O bond. The carbonyl group is having a co-planar structure & its carbon atom has sp2 hybridization.
The C=O bond breaks due to the nucleophilic attack and results in the breakage of the pi-bond. Hence, the Carbonyl carbon now becomes sp3 hybridized. The polar bond of Oxygen & Carbon atoms allow partial charges which make it easier for the nucleophile to attack at positions 1, 2.
Higher electro-negativity of the Oxygen atom in a Carbonyl compound leads to a higher density of electrons near the oxygen atom. This results in the creation of partial negative charges on the oxygen atom & partial positive charges on the Carbon atom due to which the Carbon atom behaves as an electrophile.
The negative charge on the oxygen atom can be stabilized with the introduction of any acidic group. The proton of the acidic group is added to the carbonyl oxygen atom the charge gets neutralized.
Reactions with Hydrogen Cyanide
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Hydrogen Cyanide when reacts with carbonyl groups and undergoes nucleophilic addition reaction. It generally produces cyanohydrins in the presence of a base catalyst which is used to increase the reaction rate. The CN- anion that acts as powerful nucleophile further attacks the carbonyl group and forms a new sigma bond.
C=O having a polar nature makes the nature of the carbonyl carbon electrophiles. The nucleophilic attack exerted by the cyanide anion on the carbonyl carbon forms an intermediate which is further protonated and the cyanohydrins are formed.
Nucleophilic Addition Reaction with Monohydric Alcohols
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In this type of chemical reaction, the monohydric alcohols undergo a nucleophilic addition reaction with aldehydes and ketones to form hemiacetals that further reacts with another molecule of alcohol and forms acetal (geminal diethers). An acid catalyst is required for this reaction to take place.
For Example:
Acetone (CH3COCH3) + Ethanol (C2H5OH) → 2, 2- diethoxypropane (C7H16O2)
Here the hemiacetals undergo hydrolysis to form alcohol & carbonyl compounds, the water which is formed during the process should be removed. The Nucleophilic attack in the reaction is carried out by the alcohol and the carbonyl oxygen is protonated. After this, the nucleophilic alcohol is deprotonated and the hemiacetal is formed. Repetition of the same process will form the acetal.
Other Examples of Nucleophilic Addition Reaction
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Nucleophilic Addition Reaction of Water
The chemical reaction, in which nucleophilic addition of water to a carbonyl compound (aldehyde or ketone) takes place and forms a germinal diol (hydrate). Due to neutral conditions, this is a slow reaction. However, the rate can be increased with the addition of a catalyst – either an acid or a base.
An example of such a reaction is: CH3CHO + H2O → C3H8O2
Nucleophilic Addition with Grignard Reaction
The Grignard Reagents (R-Mg-X) reacts with the carbonyl compounds and form alcohols.
The Nucleophilic atom which is present in the alkyl radical R attacks the Aldehyde or Ketone (C=O) and a single electron is transferred forming an organometallic intermediate. After which, the protonation of the alkoxide oxygen takes place and primary alcohol is formed with formaldehyde in the presence of water (R, R’ = H), with other aldehydes forms the secondary alcohol (R’=H) and with ketones forms tertiary alcohol.
Nucleophilic Addition Reaction with Primary Amines
Imines are formed when the primary amines (ArNH2 or R-NH2) undergo the nucleophilic reaction with ketone or aldehydes. Carbinolamines are formed which afterward dehydrates to form substituted imines.
Firstly, in an acidic or a base reaction, the protonation of the carbonyl group takes place in which the neutral nucleophile is more likely to attack the carbonyl compound. Then, the Nucleophile N attacks the electrophilic C of the C=O from the pi-bond. A proton gets removed which neutralizes the positive charge on the N and results in the formation of a Carbinolamine as intermediate.
After this, in a simple acid or base reaction, another protonation takes place, where the –OH group leaves. The electrons of N are used to push out the –OH group which is a neutral molecule of water (H2O), which results in the C=N bond in the form of an iminium ion. Finally, the iminium N forms the imine product through deprotonation and it regenerates the acid catalyst.
Things to Remember
- In a chemical reaction, in Organic chemistry, a nucleophilic addition reaction takes place where a chemical compound having a double bond or triple bond reacts with a nucleophile which results in the breakdown of the double or triple bond.
- The nucleophilic addition reaction takes place easily with acetone because the carbocation will form and the two CH3 groups present in acetone will stabilize it.
- The steps involved in the mechanism of Nucleophilic addition reaction of a Carbonyl compound are – Generation of Nucleophile, Nucleophilic Attack and Protonation & Regeneration of a Catalyst.
- The Carbonyl group has Sp2 hybridization which undergoes nucleophilic addition reaction. The pi bond breaks down and the carbonyl compound becomes sp3 hybridized.
- Aldehydes in comparison to the Ketones are more reactive towards the Nucleophilic Addition Reaction.
- Nucleophilic Addition Reaction is a sub-topic of Chapter – 12: Aldehydes, Ketones, and Carboxylic Acids. This particular sub-topic carries 2-3 marks and the overall chapter carries 7-8 marks according to the current marking scheme.
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Sample Questions
Ques. In which case the carboxylic acid does not show nucleophilic addition reaction? (2 Marks)
Ans. If there is resonance in between the compounds, the carboxylic acid will not show any nucleophilic addition reaction. Due to this, the lone pair of electrons on the O atom will get delocalized.
Ques. Why is Benzaldehyde less reactive to nucleophilic addition reaction than an aliphatic aldehyde? (2 Marks)
Ans. In the reaction, the Aromatic rings will form which will act as an electron-donating group because of resonance. This electron-donating group makes the benzaldehyde less electrophilic. The carbonyl atom of benzaldehyde is less positive in the aromatic aldehyde.
Ques. Which of the following will favor nucleophilic reaction the most? (3 Marks)
a) CH3- CH2 - CH2C - CH3
b) (CH3)2 C = O
c) CH3 CH2CHO
d) CH3 CHO
Ans. The correct option is d) CH3 CHO
The aldehydes are more favorable in a nucleophilic addition reaction than Ketones because the ketone are having two +I groups which will produce less positive charge on the carbonyl carbon. Now if it comes to the large chain& steric hindrance, C is having a larger chain and will release more electron and +I effect.
Hence, option d) will favor the nucleophilic reaction the most.
Ques. What does the nucleophilic addition reaction with Hydrazine form? (3 Marks)
Ans. If an Aldehyde or a Ketone undergoes nucleophilic reaction with hydrazine (NH2 - NH2), it will result in the formation of hydrazones. Also, if the reaction undergoes phenylhydrazines, it will form phenylhydrazones. The reaction that takes place is given below:
Ques. If a Ketone is treated with HCN to form Cyanohydrin, what type of reaction will it be? (3 Marks)
i) Nucleophilic Addition
ii) Nucleophilic Substitution
iii) Electrophilic Addition
iv) Electrophilic Substitution
Ans. The correct option is - (i) Nucleophilic addition reaction
The atoms of HCN are added to the C=O bond of a ketone, due to nucleophilic attack of the cyanide anion on the electrophilic carbon of the carbonyl compound.
The reaction is:
Ques. Rank the carbonyls A-D below in order of decreasing electrophilicity (reactivity with nucleophiles).
(1 = Most reactive). Explain your reasoning. (3 Marks)
Ans. The carbonyl carbon is electrophilic because it has a partial positive charge.
Electron withdrawing groups (EWG). They pull away electron density, which increases electrophilicity. So C is the most reactive.
Conversely, electron-donating groups (EDG) add electron density, and so make the carbonyl carbon less positive, and less electrophilic. Alkyl groups (carbon chains) are mildly EDG so ketone B will be less reactive than C.
Hydrogen is neither an EDG nor EWG, so the aldehyde A will be in between the B and C. Also, the hydrogen is very small, so the carbonyl carbon is easy to get to (less steric bulk to block an attack).
Ester D is the least reactive because it has a resonance (the lone pair on the oxygen gets involved).
So overall, the order from most reactive to least reactive is C > A > B > D.
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