2-methylpropene ($$\text{CH}_2=C(CH_3$)_2)$$

Vicinal dibromide ($$\text{Br-CH}_2CBr(CH_3$)_2)$$

Trans-stilbene ($$\text{PhCH=CHPh})$$

Question 1

Which reagent is used to convert a vicinal dibromide into an alkyne in the sequence shown in reaction A?

Accepted Answer

NaNH\_2 (excess) in NH\_3

Question 2

In reaction B, what is the role of sodium metal in liquid ammonia?

Accepted Answer

It reduces the alkyne to a trans-alkene via dissolving metal reduction.

Question 3

What is the major organic product when 2-bromo-2-methylpropane is treated with NaOEt/EtOH?

Accepted Answer

2-methylpropene ($$	ext{CH}_2=C(CH_3$)_2)$$

Question 4

Which step in reaction A introduces two bromine atoms across the double bond?

Accepted Answer

Step 2: Br\_2

Question 5

What is the stereochemistry of the alkene product formed in reaction B after treatment with Na/NH\_3?

Accepted Answer

Trans (E) alkene

Question 6

In reaction C, what type of reaction occurs when a secondary alcohol is treated with concentrated HCl?

Accepted Answer

SN1 substitution

Question 7

Which of the following best describes the product mixture from reaction C?

Accepted Answer

A racemic mixture of alkyl chlorides

Question 8

What is the function of NH\_4Cl (aq) in the final step of reaction A?

Accepted Answer

It neutralizes excess base and protonates the terminal alkyne.

Question 9

Which sequence of reagents would convert 2-bromo-2-methylpropane to 2-methyl-2-butene?

Accepted Answer

None of the above; the sequence forms a terminal alkyne, not an alkene

Question 10

Which intermediate is formed after the addition of Br\_2 to 2-methylpropene in reaction A?

Accepted Answer

Vicinal dibromide ($$	ext{Br-CH}_2CBr(CH_3$)_2)$$

Question 11

Why does the reaction of a secondary alcohol with concentrated HCl produce a racemic mixture?

Accepted Answer

The reaction proceeds via a carbocation intermediate, allowing attack from either side.

Question 12

Which of the following statements about dissolving metal reduction (Na/NH\_3) of alkynes is correct?

Accepted Answer

It selectively produces trans-alkenes from alkynes.

Question 13

What is the expected product when phenylacetylene is treated with Na/NH\_3?

Accepted Answer

Trans-stilbene ($$	ext{PhCH=CHPh})$$

Question 14

Which of the following is a common misconception about the use of NaNH\_2 (excess) in alkyne synthesis?

Accepted Answer

It can deprotonate terminal alkynes, allowing for further functionalization.

Question 15

In the conversion of a secondary alcohol to an alkyl chloride using HCl, which intermediate is formed?

Accepted Answer

Carbocation

Question 16

Which reagent would you use to convert a terminal alkyne to a trans-alkene?

Accepted Answer

Na/NH\_3

Question 17

What is the role of Br\_2 in the multi-step synthesis shown in reaction A?

Accepted Answer

It adds two bromine atoms across the double bond to form a vicinal dibromide.

Question 18

Which of the following best explains why a racemic mixture is formed in the SN1 reaction of a chiral alcohol with HCl?

Accepted Answer

The planar carbocation intermediate allows nucleophilic attack from either side.

Question 19

Which product is formed when 2-bromo-2-methylpropane is treated with NaNH\_2 (excess) in NH\_3 after dibromination?

Accepted Answer

Terminal alkyne ($$	ext{CH}_3C\equiv C)$$

Alkyne Synthesis and Reactions: Practice Exam Study Notes

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