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Question 1

alkyl halides

Answer 1

Alkyl Halides:
Compounds with a halogen (F, Cl, Br, or I) bonded to an sp³ hybridized carbon atom.
General formula: R-X, where R is the alkyl group and X is the halogen.

Question 2

Nucleophilic Substitution:

Answer 2

Nucleophilic Substitution:
A reaction where a nucleophile (electron-rich species) replaces a leaving group (usually a halide) in a molecule.
Two main types:
SN2: A one-step, bimolecular mechanism where bond-making and bond-breaking occur simultaneously.
Key points:
Inversion of configuration (if the carbon is chiral).
Favored by strong nucleophiles and less steric hindrance.
SN1: A two-step, unimolecular mechanism involving the formation of a carbocation intermediate.
Key points:
Racemization occurs at the carbon center (50% retention, 50% inversion).
Favored by weak nucleophiles and stable carbocations.

Question 3

Factors Affecting Substitution Reactions:

Answer 3

Factors Affecting Substitution Reactions:
Substrate: Primary carbons favor SN2, while tertiary carbons favor SN1
.
Nucleophile: Stronger nucleophiles favor SN2.
Leaving Group: Better leaving groups (weaker bases) facilitate substitution.
Solvent:
Polar protic solvents favor SN1 (e.g., water, alcohol).
Polar aprotic solvents favor SN2 (e.g., acetone, DMSO).

Question 4

What is the general formula for an alkyl halide?

Answer 4

Answer: R-X, where R is an alkyl group and X is a halogen.

Question 5

Which reaction mechanism involves a carbocation intermediate?

Answer 5

Answer: SN1 mechanism.

Question 6

In SN2 reactions, what happens to the configuration of a chiral carbon?

Answer 6

Answer: The configuration inverts (backside attack).

Question 7

What type of nucleophile is required for an SN2 reaction?

Answer 7

Answer: A strong nucleophile.

Question 8

What is a good example of a polar aprotic solvent for SN2 reactions?

Answer 8

Answer: Acetone or dimethyl sulfoxide (DMSO).

Question 9

Which type of alkyl halide is most likely to undergo an SN1 reaction?

Answer 9

Answer: A tertiary alkyl halide.

Question 10

What functional group in adrenaline makes it polar and capable of hydrogen bonding?

Answer 10

Answer: Hydroxyl (-OH) groups.

Question 11

Why is a good leaving group important in substitution reactions?

Answer 11

Answer: It stabilizes the negative charge after it leaves, making the reaction more favorable.

Question 12

When Does Racemization Occur?

Answer 12

SN1 Reaction:
In an SN1 reaction, the first step involves the formation of a carbocation intermediate after the leaving group departs.
The carbocation is planar (sp² hybridized) and lacks chirality, so the incoming nucleophile can attack from either side (top or bottom of the plane).
This leads to the formation of both enantiomers in equal proportions, causing racemization.
Conditions for Racemization:
A chiral carbon atom connected to a good leaving group (e.g., halides like Cl, Br, or I).
A reaction mechanism where a planar intermediate (e.g., a carbocation) is formed, allowing for non-stereospecific attack.

Question 13

Example: Racemization in SN1

Answer 13

Suppose you have the following chiral alkyl halide:

Starting compound: (R)−2-bromobutane
Reaction: SN1 substitution with water as the nucleophile.

Steps:

Formation of Carbocation:
The bromine atom (leaving group) departs, creating a planar carbocation at the chiral carbon center.
CH3−CH(+)−CH2−CH3
2.Nucleophilic Attack:
The water molecule (nucleophile) can attack the carbocation from either side of the planar structure.
3. Result:
This produces a racemic mixture:
(R)−2-butanol
(S)−2-butanol
Both enantiomers are formed in equal amounts.

Question 14

Visualizing Racemization:

Answer 14

Before Reaction: The molecule exists as a single enantiomer (e.g.,
(R)−).
After Reaction: Equal amounts of
(R)− and
(S)− enantiomers are formed, resulting in no net optical rotation.

Question 15

Key Points to Remember:

Answer 15

Racemization occurs when a reaction leads to the formation of a planar intermediate (like a carbocation in SN1
1).
SN2 reactions do not cause racemization; instead, they result in inversion of configuration because the nucleophile attacks from the opposite side of the leaving group.

Question 16

Why Strong Nucleophiles Favor SN2 :

Answer 16

SN2 is a bimolecular mechanism, meaning the nucleophile actively participates in the rate-determining step by attacking the electrophilic carbon from the opposite side of the leaving group.
A strong nucleophile:
Has a high electron density or charge to attack the electrophilic carbon efficiently.
Rapidly displaces the leaving group due to its high reactivity.
Examples of strong nucleophiles:
OH−, RO−, CN−, I−, and
S2−
.
Weak nucleophiles, like water (H2O) or alcohols (
ROH), are less effective because they react more slowly, giving the reaction more time to favor other pathways, such as SN1

Question 17

Why Primary Carbons Favor SN2:

Answer 17

The SN2 mechanism occurs via a single-step, concerted reaction, with the nucleophile attacking while the leaving group departs. This attack requires easy access to the electrophilic carbon.
Primary carbons:
Are less sterically hindered (fewer bulky groups around the carbon).
Provide ample space for the nucleophile to attack from the opposite side of the leaving group.
Tertiary carbons, on the other hand:
Are highly crowded due to the presence of bulky alkyl groups.
Prevent nucleophilic attack, making the SN2 mechanism unlikely. Instead, tertiary carbons favor SN1, which doesn’t require direct nucleophilic attack

Question 18

Why Polar Aprotic Solvents Favor SN2:

Answer 18

Solvents can either enhance or hinder the nucleophile’s reactivity, and polar aprotic solvents are particularly beneficial for SN2 reactions.
Polar Aprotic Solvents:
Examples: Acetone, dimethyl sulfoxide (DMSO), acetonitrile (
CH3CN), and dimethylformamide (DMF).
Do not have hydrogen atoms bonded to electronegative atoms like oxygen or nitrogen (i.e., they lack H+
for hydrogen bonding).
They solvate cations (Na+, K+), leaving the nucleophile “free” and highly reactive.
This boosts the nucleophile’s strength, allowing it to attack the electrophilic carbon more effectively.
Polar Protic Solvents:
Examples: Water, alcohols (
ROH).
Contain hydrogen atoms that form hydrogen bonds with nucleophiles.
These hydrogen bonds stabilize and “trap” the nucleophile, reducing its reactivity, which slows down the SN2 reaction.

Question 19

Key Summary:

Answer 19

Strong Nucleophiles: Enhance the nucleophilic attack on the electrophilic carbon in the single-step SN2 mechanism.
Primary Carbons: Offer minimal steric hindrance, making it easier for the nucleophile to access the electrophilic carbon.
Polar Aprotic Solvents: Keep the nucleophile free and reactive by not forming hydrogen bonds, favoring a fast and efficient nucleophilic substitution.