Video Transcript
During the addition reactions of an
unsymmetrical alkene and hydrogen chloride, the hydrogen atom of the HCl molecules
usually adds to the carbon atom in the alkene molecule that has the greatest number
of hydrogen atoms. Which of the following scientists
proposed a rule that can be used to predict this observation? (A) Baeyer, (B) Kekulé, (C) Hund,
(D) Markovnikov, (E) Faraday.
To begin, an addition reaction is a
type of chemical reaction in which two or more reactant molecules combine to make a
single type of molecular product. Alkenes can undergo different types
of addition reactions. These include hydrogenation,
hydrohalogenation, halogenation, and hydration.
We can classify the addition
reaction involving an alkene and hydrogen chloride as a hydrohalogenation
reaction. In a hydrohalogenation reaction, an
alkene reacts with a hydrogen halide, such as hydrogen chloride, to make a new
larger molecule known as a haloalkane.
The hydrohalogenation reaction
described in the problem is between an unsymmetrical alkene and hydrogen
chloride. Therefore, the type of haloalkane
that would be produced is a chloroalkane. But-1-ene is an example of an
unsymmetrical alkene. When but-1-ene reacts with hydrogen
chloride, the 𝜋 bond in the alkene and the single bond in hydrogen chloride
break. This allows for the formation of a
new carbon–hydrogen bond and a new carbon–chlorine bond. But the hydrogen atom can form a
new bond with either one of the carbon atoms in the carbon–carbon double bond.
If the hydrogen atom forms a bond
with the first carbon atom and the chlorine atom forms a bond with the second
carbon, then 2-chlorobutane forms. But if the chlorine atom forms a
bond with the first carbon atom and the hydrogen atom forms a bond with the second
carbon, then 1-chlorobutane forms.
The following rule can be used to
predict the major or most likely product. The hydrogen atom of the H-X
molecule adds to the carbon atom of the carbon–carbon double bond that has more
hydrogen substituents. The name of the scientist who
proposed this rule is Vladimir Markovnikov. And so the name of the rule is
known as Markovnikov’s rule. If we follow Markovnikov’s rule,
then the hydrogen atom of hydrogen chloride will add to carbon number one because it
has two covalently bonded hydrogen atoms, whereas carbon number two only has
one. This means the chlorine atom will
add to carbon number two. Therefore, 2-chlorobutane is the
major product of this reaction.
In conclusion, the scientist who
proposed a rule that can be used to predict the major product of the reactions of
unsymmetrical alkenes with hydrogen halides is Markovnikov, or answer choice
(D).
