Alcohol Reactions with Hydrogen Halides

Questions and Answers

What happens to the OH group when alcohols react with HX?

It converts into a ketone.

It remains unchanged.

It gets replaced by the X atom. (correct)

It is converted to H2O.

Under which conditions do alcohols proceed by the SN2 mechanism?

When the OH is in a cyclic alcohol.

When the OH is attached to a tertiary carbon.

When the OH is attached to a CH3 or a 1° carbon. (correct)

When the OH is attached to an aromatic ring.

Which of the following halides can replace the OH group in alcohols?

Br only

I only

Cl, Br, or I (correct)

Cl only

What is the main functional group being replaced in this reaction?

Hydroxyl group Signup and view all the answers

Which of the following statements about the addition of HX to alcohols is false?

The reaction can occur with any alcohol. Signup and view all the answers

What type of carbon does the OH group need to be bonded to for the reaction to proceed via the S_N1 mechanism?

Both B and C Signup and view all the answers

The OH group is converted into Br- in this reaction.

False Signup and view all the answers

What is formed after the protonation of the OH group in this reaction?

Water (OH2+) Signup and view all the answers

In the reaction, the positively charged species formed after protonation of the OH group is called a ______.

carbocation Signup and view all the answers

Match the following components of the reaction with their roles:

OH = Leaving group after protonation HBr = Source of bromide ion Br- = Final product R-C-R = Substrate undergoing reaction Signup and view all the answers

Study Notes

Reaction of Alcohols with HX

Alcohols react with hydrogen halides (HX), where X can be chlorine (Cl), bromine (Br), or iodine (I).
During this reaction, the hydroxyl group (OH) in alcohols is substituted by the halogen (X).
The mechanism of substitution depends on the structure of the alcohol.

SN2 Mechanism

If the alcohol has the hydroxyl group attached to a methyl group (CH3) or a primary (1°) carbon, the reaction proceeds via an SN2 mechanism.
SN2 (bimolecular nucleophilic substitution) features a single concerted step where the nucleophile (the halide ion from HX) attacks from the opposite side of the leaving group (the OH group), leading to inversion of configuration.

Key Points

The choice of halide significantly affects the reactivity and strength of the bond formed with the alcohol.
The structure of the alcohol (primary vs. tertiary) influences whether SN2 or other mechanisms (like SN1) will be favored in the reaction.

SN1 Reaction Mechanism

SN1 reactions occur when the hydroxyl (OH) group is attached to a secondary (2°), tertiary (3°), or stabilized carbon atom.
The reaction involves the formation of a carbocation intermediate after the protonation of the OH group.

Chemical Reaction Overview

Hydroxide ion (OH-) reacts with hydrogen bromide (HBr) to produce a bromide compound.
The tertiary carbon structure (R-C-R) has an OH group that undergoes protonation, resulting in water (H2O) and a carbocation.
The positively charged carbocation is a key intermediate that allows for the subsequent departure of the bromide ion (Br-) as a leaving group.

Reaction Steps

Protonation of the OH group leads to the formation of water (OH2+), facilitating the carbocation formation.
The carbocation is unstable and reacts quickly with the leaving group (Br-).
The final product includes the formation of a carbon-bromine bond (R-C-Br) along with the release of Br-.

Key Characteristics

Tertiary carbocations are more stable due to hyperconjugation and inductive effects, making them favorable for SN1 pathways.
The formation of the carbocation is a rate-determining step, which is why the reaction has characteristic kinetics of uniphasic (SN1) mechanisms.

Conclusion

Understanding the conditions under which SN1 mechanisms occur is crucial for predicting reaction outcomes in organic chemistry.

Description

Explore the reaction mechanisms of alcohols with hydrogen halides (HX) in this quiz. Learn about the SN2 mechanism and how the structure of the alcohol influences the substitution process. Test your knowledge on the role of halides in these reactions.