Q33E - Organic Chemistry (Mcmurry)

Question

Predict the products from reaction of 2-hexyne with the following

reagents:

1. 2 equiv ${\mathbf{Br}}_{\mathbf{2}}$ 
2. 1 equiv HBr
3. Excess HBr
4. Li in  ${\mathbf{NH}}_{\mathbf{3}}$
5.  ${\mathbf{H}}_{\mathbf{2}}\mathbf{O}$, ${\mathbf{H}}_{\mathbf{2}}{\mathbf{SO}}_{\mathbf{4}}$, ${\mathbf{HgSO}}_{\mathbf{4}}$ 

Step-by-Step Solution

Answer

(a) 

(b) 

(c) 

(d) 

(e) 

Step 1: Reaction of 2-hexyne with 2 equivalents of Br 2

The treatment of 2-hexyne with two equivalents of bromine results in a tetrabromoalkane. When one equivalent of bromine is used, dibromoalkane is produced as a product.

Step 2: Reaction of 2-hexyne with 2 equivalents of Br 2

(a)The reaction of 2-hexyne with two equivalents of ${\text{Br}}_{\text{2}}$  produces 2,2,3,3-tetrabromohexane as the product. The reaction can be given as:

Step 3: Reaction of alkyne with 1 equivalent of HBr

The reaction of an alkyne with one equivalent of HBr leads to a trans alkene. These sorts of reactions are classed as addition reactions.

Step 4: Reaction of 2-hexyne with 1 equivalent of HBr

(b)The reaction of 2-hexyne with 1 equivalent of HBr produces (Z)-2-bromo-hex-2-ene and (Z)-3-bromo-hex-2-ene as the product. The reaction can be given as:

Step 5: Reaction of alkyne with excess HBr

The reaction of an alkyne with excess HBr produces a dibromoalkane as the product. The addition of HBr to an alkyne forms a vinylic carbocation which further forms a vinylic bromide.

Step 6: Reaction of 2-hexyne with excess HBr

(c)The reaction of 2-hexyne with HBr produces 3,3-dibromohexane and 2,2-dibromohexane as the products. The reaction can be given as:

Reaction (c)

Step 7: Reaction of alkyne with Li in NH 3

Another method for converting alkynes to alkenes is by the reagent lithium in ammonia. This method produces trans alkenes.

Step 8: Reaction of 2-hexyne with Li in NH 3

(d)The reaction of 2-hexyne with Li in ${\text{NH}}_{\text{3}}$  produces (E)-hex-2-ene as the product. The reaction can be given as:

Step 9: Reaction of alkyne with H 2 O , H 2 SO 4 and HgSO 4

The oxidation of alkyne occurs when a particular alkyne is treated with water, sulphuric acid and mercury(II) sulfate. The hydration of an alkyne in the presence of aqueous acid and mercury(II)sulfate leads to a methyl ketone.

Step 10: Reaction of 2-hexyne with H 2 O , H 2 SO 4 and HgSO 4

(e)The reaction of 2-hexyne with  ${\text{H}}_{\text{2}}\text{O}$, ${\text{H}}_{\text{2}}{\text{SO}}_{\text{4}}$ and ${\text{HgSO}}_{\text{4}}$  produces hexan-2-one and hexan-3-one as the products. The reaction can be given as: