Question

How would you synthesize each of the following? a. 1,2 -dibromopropane from propene b. acetone (2-propanone) from an alcohol c. tert-butyl alcohol (2-methyl-2-propanol) from an alkene (See Exercise 62.) d. propanoic acid from an alcohol

Short answer

To synthesize 1,2-dibromopropane from propene, perform electrophilic addition with Br2 to obtain 2-bromopropane, and then nucleophilic substitution with Br- to form 1,2-dibromopropane. To synthesize acetone from an alcohol, oxidize isopropyl alcohol using K2Cr2O7 or KMnO4. For tert-butyl alcohol from an alkene, hydrate 2-methylpropene with strong acid. Lastly, to synthesize propanoic acid from an alcohol, oxidize 1-propanol using K2Cr2O7 or KMnO4 to form propanoic acid.

Step-by-step solution

Bromination of propene

To synthesize 1,2-dibromopropane from propene, we must first brominate the double bond of the propene molecule. This can be done by adding bromine (Br2) and performing an electrophilic addition reaction. The reaction will follow Markovnikov's rule, adding the first bromine atom to the more substituted carbon, creating a 2-bromopropane intermediate.

Nucleophilic substitution of 2-bromopropane

To add the second bromine atom to the molecule, we must perform a nucleophilic substitution reaction. This involves treating the 2-bromopropane intermediate with a good nucleophile such as the bromide ion (Br-) in the presence of a polar aprotic solvent. This will result in the formation of 1,2-dibromopropane. b. Synthesis of acetone (2-propanone) from an alcohol

Oxidation of isopropyl alcohol

To synthesize acetone from an alcohol, we must perform oxidation of the alcohol group. Specifically, we want to oxidize isopropyl alcohol (2-propanol). This can be achieved by treating the alcohol with an oxidizing agent, such as potassium dichromate (K2Cr2O7) or potassium permanganate (KMnO4). This will yield acetone (2-propanone) as the main product. c. Synthesis of tert-butyl alcohol (2-methyl-2-propanol) from an alkene

Hydration of the alkene

To synthesize tert-butyl alcohol from an alkene, we'll start with 2-methylpropene (isobutylene) as our starting material. We must perform a hydration reaction, which involves adding water to the double bond in the presence of a strong acid catalyst (concentrated sulfuric acid or phosphoric acid). This reaction follows Markovnikov's rule and will produce tert-butyl alcohol (2-methyl-2-propanol). d. Synthesis of propanoic acid from an alcohol

Oxidation of 1-propanol

To synthesize propanoic acid from an alcohol, we must perform the oxidation of 1-propanol (n-propyl alcohol). This can be achieved by treating the alcohol with a strong oxidizing agent, such as potassium dichromate (K2Cr2O7) or potassium permanganate (KMnO4). After the oxidation reaction, the primary alcohol is first converted to an aldehyde (propanal) and then to propanoic acid (a carboxylic acid) as the final product.

Key concepts

Electrophilic Addition Reaction

An electrophilic addition reaction is an essential reaction mechanism in organic chemistry, especially when dealing with alkenes. Alkenes are hydrocarbons that contain carbon-carbon double bonds, which are rich in electrons. These electron-rich double bonds can attract electrophiles, which are electron-deficient species, to add across the double bond.

In the synthesis of 1,2-dibromopropane from propene, bromine ( Br_2 ) acts as an electrophile. When bromine approaches the propene, the electron density from the double bond causes the bromine molecule to polarize, with one end becoming positive. This polarized bromine then attacks the double bond, forming a bromonium ion intermediate.

• Initial bromine addition follows Markovnikov's rule, indicating the more substituted carbon is bonded first due to stability reasons.
• The reaction can be facilitated in non-aqueous solutions to avoid competing reactions.

Consequently, the reaction results in the addition of a bromine atom to each carbon of the double bond, leading to the formation of 1,2-dibromopropane.

Oxidation of Alcohols

Oxidation of alcohols is a significant reaction in organic chemistry for transforming alcohols into other functional groups. The degree of oxidation an alcohol can undergo depends on its structure. Primary, secondary, and tertiary alcohols behave differently during oxidation.

In the case of transforming isopropyl alcohol into acetone, a secondary alcohol undergoes oxidation. With agents like potassium dichromate ( K_2Cr_2O_7 ) or potassium permanganate ( KMnO_4 ), the hydroxyl group is oxidized to a carbonyl group, yielding acetone.

• Using appropriate oxidizing agents ensures complete and safe oxidation.
• Controlling reaction conditions, such as temperature and using either acidic or basic media, is crucial for desired selectivity.

This selective oxidation is possible because secondary alcohols are ideal for ketone formation, making acetone the primary product in this process.

Nucleophilic Substitution

Nucleophilic substitution is a reaction mechanism where a nucleophile, which is a species with lone pair electrons, substitutes a leaving group in a molecule. This reaction is critical in forming various organic compounds.

To synthesize 1,2-dibromopropane, the second bromine atom is introduced via a nucleophilic substitution reaction. After electrophilic addition creates a 2-bromopropane intermediate, bromide ion ( Br^- ) acts as a nucleophile in a polar aprotic solvent such as acetone.

• Polar aprotic solvents are preferred as they stabilize the nucleophile and enhance the reaction rate by not forming hydrogen bonds with them.
• This reaction is often SN2 (bimolecular nucleophilic substitution), where the nucleophile attacks the carbon bearing the leaving group directly.

The nucleophilic attack leads to the substitution of hydrogen on the second carbon with a bromine atom, consistently yielding 1,2-dibromopropane.

Hydration of Alkenes

Hydration of alkenes is a fundamental organic reaction that introduces a hydroxyl group ( OH ) into a molecule, effectively transforming an alkene into alcohol. This reaction is particularly useful for synthesizing alcohols from simple alkenes.

The key to synthesizing tert-butyl alcohol from isobutylene (2-methylpropene) is the addition of water across the double bond. This reaction requires a strong acid catalyst, such as sulfuric or phosphoric acid, to facilitate the process.

• According to Markovnikov's rule, the water molecule adds such that the hydrogen atom bonds with the less substituted carbon, and the hydroxyl group bonds with the more substituted carbon.
• This reaction offers high regioselectivity, ensuring a specific structure for the desired alcohol.

The product, tert-butyl alcohol, results from an efficient and regioselective hydration, proving valuable in various applications.