Most Important Alkyl Halides MCQs with Answers | Chemistry

The electronegativity of Iodine is 2.5.

On the basis of electronegativity difference alone, alkyl iodides would be the least reactive if an electrophile were the attacking reagent. However, this is contradicted by the dominant role of bond energy in determining overall reactivity.

Nucleophilic substitution reactions on alkyl halides primarily involve the breaking of the carbon-halogen (C-X) bond and the formation of a new carbon-nucleophile (C-Nu) bond.

In IUPAC nomenclature, halogen atoms are treated as substituents. The parent alkane is methane, and the chlorine is the substituent.

The longest continuous carbon chain is four carbons (butane), and the chlorine is on the first carbon.

The longest chain is butane. Numbering from the end that gives the chlorine the lowest number places it on the second carbon.

The longest continuous carbon chain is three carbons (propane) with a methyl group on the second carbon. The chlorine is on the first carbon.

The longest continuous carbon chain is three carbons (propane). There is a methyl group and a chlorine atom both attached to the second carbon.

The mechanism of nucleophilic substitution reactions depends on whether the C-X bond breaks before or simultaneously with the C-Nu bond formation.

If the C-X bond breaks at the same time as the C-Nu bond forms, the mechanism is known as SN2.

When the C-X bond breaks initially to form a carbocation, and then the new bond is formed, it is an SN1 mechanism.

SN2 is a concerted, single-step mechanism where bond breaking and bond formation occur simultaneously.

In SN2 reactions, the attacking nucleophile approaches from the backside, opposite to the leaving group, to minimize steric hindrance and facilitate the departure of the leaving group.

The longest continuous carbon chain is two carbons (ethane), and the bromine is a substituent.

The longest chain is propane. The carbon atom bearing the functional group (chlorine) gets the lowest possible number, which is 1.

The longest chain is propane. The chlorine is on the second carbon, giving it the lowest possible number.

In an SN2 reaction, the substrate carbon atom changes its hybridization from tetrahedral sp³ to a planar sp² transition state, allowing the nucleophile to approach.

SN2 reactions proceed with an inversion of configuration at the reacting carbon center, often referred to as 'Walden inversion'.

Molecularity of a reaction is defined as the number of molecules taking part in the rate determining step.

Since an SN2 reaction is a single-step mechanism where two molecules (alkyl halide and nucleophile) participate in the rate-determining step, its molecularity is two.

An SN1 reaction is unimolecular in its rate-determining step, involving only the alkyl halide molecule.

The reactivity order for SN2 reactions is primary > secondary > tertiary, primarily due to steric hindrance. Primary alkyl halides offer the least steric hindrance, allowing the nucleophile to approach more easily.

Methyl bromide (CH₃Br) is a primary halide (even simpler than typical primary), offering the least steric hindrance for an SN2 reaction.

SN1 reactions proceed via a two-step mechanism involving the formation of a carbocation intermediate in the slow, rate-determining step.

The reactivity order for SN1 reactions is tertiary > secondary > primary, because the stability of the carbocation intermediate increases in this order (tertiary carbocations are the most stable).

Polar protic solvents stabilize the carbocation intermediate and the leaving group through solvation, thereby favoring SN1 reactions.

Strong nucleophiles are generally required for SN2 reactions to effectively attack the substrate carbon and displace the leaving group in a single step.

Increased steric hindrance around the carbon atom bearing the leaving group hinders the approach of the nucleophile, thus decreasing the rate of SN2 reactions.

Racemization is the formation of equal amounts of enantiomers (a racemic mixture). In SN1 reactions, because the carbocation intermediate is planar, the nucleophile can attack from either side, leading to racemization if a chiral center is involved.

A nucleophile is an electron-rich species that donates an electron pair to an electron-deficient center. Water (H₂O) has lone pairs on the oxygen atom, making it a nucleophile.

β-elimination reactions, also known as 1,2-elimination reactions, involve the removal of a halogen atom from the α-carbon and a hydrogen atom from the β-carbon, resulting in the formation of an alkene. This process is often called dehydrohalogenation.

Similar to nucleophilic substitution, β-elimination reactions can proceed through two main mechanisms: E1 (unimolecular elimination) and E2 (bimolecular elimination).

In an E2 reaction, a strong base abstracts a proton from the β-carbon simultaneously with the departure of the leaving group and the formation of a double bond.

The E2 reaction is a single-step, concerted mechanism involving both the alkyl halide and the base in the rate-determining step, hence its molecularity is two.

The E2 reaction proceeds through a single transition state where the C-H bond and C-X bond are simultaneously breaking, and a new C=C double bond is forming.

E2 reactions typically require the hydrogen and the leaving group to be anti-periplanar (180° dihedral angle) to allow for proper orbital overlap during the concerted elimination.

The reactivity order for E2 reactions generally follows tertiary > secondary > primary, as the more substituted alkenes formed are more stable, and tertiary halides often experience more steric hindrance with the base, favoring elimination over substitution.

In an E1 reaction, the slow, rate-determining step is the unimolecular dissociation of the alkyl halide to form a carbocation and a halide ion.

E1 reactions are two-step processes: first, the formation of a carbocation, and second, the deprotonation of the carbocation by a weak base to form an alkene.

Polar protic solvents stabilize the carbocation intermediate formed in the first step of an E1 reaction, similar to SN1 reactions.

E1 reactions favor the formation of more stable carbocations, so the reactivity order is tertiary > secondary > primary alkyl halides.

E1 reactions are typically carried out with weak bases, as a strong base would favor the E2 mechanism or SN2.

Zaitsev's Rule states that in an elimination reaction, the major product is the most substituted alkene (the one with the most alkyl groups attached to the double-bonded carbons).

High temperatures generally favor elimination reactions due to the entropic advantage of forming more molecules (alkene + HX). A strong base particularly favors E2.

Bulky bases tend to favor elimination (E2) over substitution (SN2) because steric hindrance makes nucleophilic attack at the carbon difficult, but proton abstraction from the periphery is still possible.