[orgoclear]

See the attached fig.
What is the major product?

I thought that since alkenes are more reactive than alkynes towards electrophilic substitution, it will break first. However there is no possibility of any conjugation hence the Cl should add to the other carbon.

However the answer was given as the pi bond (one of them) of the alkyne breaking and then addition of Cl- to the second carbon

Is it correct

[orgoclear]

is this so tough or so easy that no one wants to answer it?

[jdmfob]

The products of these additions are themselves substituted alkenes and can therefore undergo further addition. Because of their high electronegativity, halogen substituents on a double bond act to reduce its nucleophilicity, and thereby decrease the rate of electrophilic addition reactions.

Consequently, there is a delicate balance as to whether the product of an initial addition to an alkyne will suffer further addition to a saturated product. Although the initial alkene products can often be isolated and identified, they are commonly present in mixtures of products and may not be obtained in high yield.

[orgoclear]

The question clearly mentions that one equivalent of HCl is to be used ..

[orgopete]

is this so tough or so easy that no one wants to answer it?

It is one thing to know the answer and another to make a spurious guess. I didn't know the answer so I looked it up. From Wikipedia, vinyl acetylene,

At one time, chloroprene (2-chloro-1,3-butadiene), an industrially important monomer, was produced via the intermediacy of vinyl acetylene. In this process, acetylene is dimerized to give vinyl acetylene, which is then combined with hydrogen chloride to give 4-chloro-1,2-butadiene, which, in the presence of cuprous chloride, rearranges to 2-chloro-1,3-butadiene.

This is an interesting addition of HCl. With butadiene, it is usually argued that a kinetic product can be isolated by performing the reaction at low temperature. By doing so, the chloride adds to the most stable secondary carbocation to give the 1,2-addition product (3-chlorobut-1-ene). At higher temperature, this compound can reform the carbocation and addition of chloride to the less stable primary carbocation, but more substituted alkene leads to a thermodyanamic product, the 1,4-addition product (1-chlorobut-2-ene). This example must not have significant attack at the intermediate vinyl cation as the vinyl chloride should be stable.

The reaction: