Draw the structure of R-3-bromo-2-methylpentane

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Draw the structure of R-3-bromo-2-methylpentane. Clearly show the ranking of the groups on the chiral carbon. Design a two step synthesis that converts 2-pentanone into iso-pentyl propanoate as shown below. H2 CH3 CH3 A H2 ??? ???? ????? B H2C C- H2 H2 H2 Draw the structure of molecule B and write the reagents A and C for the two steps in the multi-step synthesis. There are the possible products of the addition reaction of 2-dimethylserene with Carey chloro 24-dimethyertare and chloro-24 dimethylpentane. However, Markoko’s rule states that the only product is 2-chloro-24 dimethypertane CHCH CHECHE HCI , – HCH CH H2 ???? “CH; ?, ? Draw the accepted me for a reaction, and explain why only this product is formed. The nucleophilie substitution reaction of this enantiomer of 3-bromohexane with chloride yields 1 or 2 stereoisomers of 3-chlorhexane as the product Experimental data on the rate of reaction concluded that the rate law was overall second order, dependent on the 3-bromohexane concentration and chloride concentration HC From this information state the probable mechanism as either Stor S2? Draw the mechanism you have chosen for this reaction as it applies to this reaction The product(s) drawn in the mechanism must clearly show the configuration at the chiral carbon

The Correct Answer and Explanation is :

Let’s break down each part of your question:

1. Structure of R-3-Bromo-2-methylpentane

The structure of R-3-bromo-2-methylpentane has the following configuration:

       CH3
        |
CH3-CH-CH2-CH2-Br
        |
       CH2

In this molecule, the chiral center is at carbon 3 (where the bromine atom is attached). To determine the configuration (R or S), we rank the substituents around the chiral center based on the Cahn-Ingold-Prelog priority rules:

  1. Bromine (Br): The atomic number of Br (35) is higher than that of carbon (6), so bromine has the highest priority (1).
  2. Ethyl group (CH2-CH3): This has the next highest priority (2).
  3. Methyl group (CH3): This has the next priority (3).
  4. Hydrogen: This is the lowest priority (4).

Using the standard convention of drawing the lowest priority group (hydrogen) pointing away from us, we determine that the configuration is R based on the arrangement of the groups.

2. Two-Step Synthesis from 2-Pentanone to Iso-pentyl Propanoate

For the synthesis, starting from 2-pentanone and aiming for iso-pentyl propanoate, the following sequence of reactions can be used:

Step 1: Reduction to Iso-pentanol

Reagent A: NaBH4 (Sodium borohydride) – Reduces the carbonyl group of 2-pentanone to the corresponding alcohol (iso-pentanol).

Reaction:
2-Pentanone → Iso-pentanol (C5H11OH)

Step 2: Esterification to Iso-pentyl Propanoate

Reagent C: Propanoic acid (C2H5COOH), in the presence of an acid catalyst (e.g., sulfuric acid) for esterification.

Reaction:
Iso-pentanol + Propanoic acid → Iso-pentyl propanoate

The structure of molecule B is Iso-pentyl propanoate:

CH3CH2COOCH2CH2CH2CH3

3. Addition Reaction and Markovnikov’s Rule

The reaction of 2-dimethylserene with Carey chloro-24-dimethyltarene or chloro-24-dimethylpentane follows Markovnikov’s rule, which states that in the addition of HX (like HCl) to an alkene, the proton (H) will add to the carbon with the greater number of hydrogens (leading to the more stable carbocation intermediate).

For example, the reaction of 2-dimethylserene with HCl will lead to the formation of 2-chloro-24-dimethylpentane, where the proton adds to the carbon of the alkene that has more hydrogens, leading to the formation of the more stable carbocation.

This reaction follows Markovnikov’s rule and the major product is 2-chloro-24-dimethylpentane.

4. Mechanism for Nucleophilic Substitution (S_N2 or S_N1)

The reaction of 3-bromohexane with chloride ion yields 3-chlorohexane, which can have one of two possible stereoisomers (depending on whether the reaction follows S_N1 or S_N2).

Given that the rate law is second order, dependent on both the concentration of 3-bromohexane and chloride, the most likely mechanism is S_N2. This implies that the reaction proceeds via a single-step mechanism with a backside attack by the chloride ion, leading to inversion of configuration at the chiral center.

S_N2 Mechanism:

  1. The chloride ion attacks the carbon bonded to the bromine (the electrophilic carbon) from the opposite side of the leaving group (bromine).
  2. As the chloride ion approaches, the bromine leaves, and the configuration at the chiral center is inverted.
  3. The final product is 3-chlorohexane, with the configuration at the chiral carbon inverted relative to the starting material.

The configuration change in the product reflects the typical S_N2 mechanism where the nucleophile attacks the carbon from the opposite side to the leaving group, resulting in a 180° inversion of configuration.

Summary:

  1. R-3-bromo-2-methylpentane is drawn, with priority groups determined for its chiral center.
  2. Two-step synthesis: Reduction of 2-pentanone to iso-pentanol (reagent A: NaBH4) followed by esterification with propanoic acid (reagent C).
  3. Addition reaction follows Markovnikov’s rule, leading to 2-chloro-24-dimethylpentane as the product.
  4. The S_N2 mechanism explains the inversion of configuration in the nucleophilic substitution of 3-bromohexane with chloride.

Let me know if you need any additional clarification!

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