Question

Propose a synthesis of each of the following compounds starting with 2 -bromobutane. If you propose a reaction that will give a mixture of products, be sure to indicate all products from that reaction. (a) 2-butene (b) 2-butanol (c) butane (d) 2-methoxybutane

Short answer

To synthesize the compounds: (a) 2-butene, use an elimination reaction; (b) 2-butanol, use a substitution with aqueous NaOH; (c) butane, perform hydrogenation; (d) 2-methoxybutane, use substitution with CH3OH.

Step-by-step solution

Understanding the Starting Material

First, identify the structure of 2-bromobutane. It is a four-carbon chain with a bromine atom attached to the second carbon: CH3-CH(Br)-CH2-CH3.

Synthesis of 2-butene

To produce 2-butene from 2-bromobutane, perform a dehydrohalogenation reaction (elimination reaction). Use a strong base such as NaOH or KOH in an alcoholic solvent. This removes the bromine and a hydrogen atom from adjacent carbon atoms, forming a double bond, resulting in either of the following products: Trans-2-butene: CH3-CH=CH-CH3 Cis-2-butene: CH3-CH=CH-CH3

Synthesis of 2-butanol

To synthesize 2-butanol from 2-bromobutane, carry out a nucleophilic substitution reaction. Use aqueous NaOH or KOH, which will replace the bromine atom with an OH group: CH3-CH(OH)-CH2-CH3 (2-butanol).

Synthesis of butane

To convert 2-bromobutane to butane, perform a hydrogenation reaction. Use a palladium or nickel catalyst with hydrogen gas. This will replace the bromine atom with a hydrogen atom: CH3-CH2-CH2-CH3 (butane).

Synthesis of 2-methoxybutane

To synthesize 2-methoxybutane, carry out a nucleophilic substitution reaction with methanol (CH3OH). The methanol will replace the bromine atom with a methoxy group: CH3-CH(OCH3)-CH2-CH3 (2-methoxybutane).

Key concepts

Dehydrohalogenation Reaction

To transform 2-bromobutane into 2-butene, we use a process called dehydrohalogenation. This is an elimination reaction where a base removes a hydrogen atom together with the bromine atom. This forms a double bond between two carbon atoms.

Typically, strong bases like NaOH or KOH in an alcoholic solution, such as ethanol, are used for this process. For instance:

• The bromine is removed from the second carbon atom, and one hydrogen is removed from an adjacent carbon, creating a double bond.
• The result can be either trans-2-butene or cis-2-butene, which differ by the spatial arrangement of substituents around the double bond.

The chemical reactions can be written as:


CH3-CH(Br)-CH2-CH3 (2-bromobutane) + NaOH → CH3-CH=CH-CH3 (2-butene) + H2O + NaBr

Nucleophilic Substitution

To convert 2-bromobutane into 2-butanol or 2-methoxybutane, we use nucleophilic substitution. This type of reaction involves a nucleophile replacing a leaving group (in this case, bromine). The nucleophile could be an OH⁻ from NaOH (for 2-butanol) or a methoxy (OCH3−) from methanol (for 2-methoxybutane).

For the synthesis of 2-butanol:

• Mixing 2-bromobutane with aqueous NaOH allows the OH⁻ to displace the bromine atom.
• The OH⁻ takes the place of the bromine, resulting in 2-butanol (CH3-CH(OH)-CH2-CH3).

For the synthesis of 2-methoxybutane:

• React 2-bromobutane with methanol (CH3OH).
• The methoxy group (-OCH3) replaces the bromine, resulting in 2-methoxybutane (CH3-CH(OCH3)-CH2-CH3).

The equations for these reactions can be represented as:

CH3-CH(Br)-CH2-CH3 + NaOH → CH3-CH(OH)-CH2-CH3 + NaBr

and

CH3-CH(Br)-CH2-CH3 + CH3OH → CH3-CH(OCH3)-CH2-CH3 + HBr

Hydrogenation Reaction

To convert 2-bromobutane into butane, a hydrogenation reaction is used. Hydrogenation involves adding hydrogen atoms to a molecule, typically in the presence of a catalyst.

For converting 2-bromobutane to butane:

• Use a palladium (Pd) or nickel (Ni) catalyst along with hydrogen gas (H2).
• The bromine atom is replaced by a hydrogen atom, resulting in the formation of butane (CH3-CH2-CH2-CH3).

This reaction is particularly useful for removing functional groups and saturating hydrocarbons.

The reaction can be shown as:

CH3-CH(Br)-CH2-CH3 + H2 (with Pd or Ni catalyst) → CH3-CH2-CH2-CH3 + HBr

2-Bromobutane

2-Bromobutane is an organic compound with the chemical formula CH3-CH(Br)-CH2-CH3. It is a four-carbon chain where a bromine atom is attached to the second carbon.

In organic synthesis,

• it serves as a versatile starting material.
• It can undergo various reactions like dehydrohalogenation, nucleophilic substitution, and hydrogenation.
• Each reaction can provide a different product, enabling the synthesis of diverse organic compounds.

In understanding and using 2-bromobutane for synthesis, it is crucial to know the different reaction conditions and reactants required for each specific product aim.
For instance:

• Dehydrohalogenation with a strong base in alcohol to form alkenes.
• Nucleophilic substitution with appropriate nucleophiles to form alcohols or ethers.
• Hydrogenation with hydrogen gas and a catalyst to form alkanes.

These reactions highlight the importance of 2-bromobutane in organic chemistry.