Question

(a) Draw the structures of ethyl, n-propy1, isobutyl, and neopentyl bromides. These structures can be considered methyl bromide with one of its hydrogens replaced by various alky1 groups \(\left(\mathrm{GGH}_{2} \mathrm{Br}\right)\). What is the group \(\mathrm{G}\) in each case? (b) The relative rates of reaction (with ethoxide ion) are roughly: methyl bromide, \(100 ;\) ethyl bromide, \(6 ; \mathrm{n}\) -propyl bromide, \(2 ;\) isobutyl bromide, \(0.2 ;\) neopentyl bromide, \(0.00002\). What is the effect of the size of the group \(G\) attached to carbon bearing the halogen?

Short answer

The group G in each case is as follows: Ethyl bromide (\(CH_{2}CH_{3}\)), n-Propyl bromide (\(CH_{2}CH_{2}CH_{3}\)), Isobutyl bromide (\(CH_{2}CH(CH_{3})CH_{3}\)), and Neopentyl bromide (\(C(CH_{3})_{3}\)). As the size of group G increases, the relative rate of reaction with ethoxide ion decreases due to steric hindrance, making it more difficult for the ethoxide ion to approach the carbon atom carrying the halogen.

Step-by-step solution

(Step 1: Draw Structures and Identify Group G)

: (a) 1. Ethyl bromide (\(CH_{3}CH_{2}Br\)) Group G: \(CH_{2}CH_{3}\) (ethyl group) 2. n-Propyl bromide (\(CH_{3}CH_{2}CH_{2}Br\)) Group G: \(CH_{2}CH_{2}CH_{3}\) (n-propyl group) 3. Isobutyl bromide (\(CH_{3}CH(CH_{3})CH_{2}Br\)) Group G: \(CH_{2}CH(CH_{3})CH_{3}\) (isobutyl group) 4. Neopentyl bromide (\(CH_{3}C(CH_{3})_{3}Br\)) Group G: \(C(CH_{3})_{3}\) (neopentyl group) In phase 2, we will discuss the effect of the size of group G on the relative rates of reaction with ethoxide ion.

(Step 2: Analyze the Relative Rates of Reaction)

: (b) The relative rates of reaction (with ethoxide ion) are: Methyl bromide: 100 Ethyl bromide: 6 n-Propyl bromide: 2 Isobutyl bromide: 0.2 Neopentyl bromide: 0.00002 As the size of group G increases, we observe a decrease in the relative rate of reaction. This can be explained by considering steric hindrance. With a larger group G, it becomes more difficult for the ethoxide ion to approach the carbon atom carrying the halogen (bromide in this case), which decreases the likelihood of a successful collision and thus reduces the reaction rate.

Key concepts

Alkyl Halides Structure

Alkyl halides, also known as haloalkanes, are a group of chemical compounds comprised of alkanes with one or more hydrogen atoms replaced by halogen atoms (fluorine, chlorine, bromine, or iodine). The general formula for an alkyl halide is \( R-X \), where \( R \) represents an alkyl group—a chain of carbon atoms bonded together—and \( X \) represents the halogen.

Considering the structures of various alkyl bromides, the alkyl group \( G \) greatly affects the properties and reactivity of the molecule. For example, ethyl bromide has a simple two-carbon chain as its alkyl group (ethyl), while more complex structures like neopentyl bromide have branched chains. The ability of the various alkyl groups to influence reactivity is intrinsic to the study of organic chemistry, especially when looking into how these molecules interact during chemical reactions.

Steric Hindrance

In organic chemistry, steric hindrance refers to the restriction of chemical reactions by the physical size of groups within a molecule. As the groups attached to a central atom become larger, they take up more space and can impede the approach of reactants towards reactive sites on the molecule.

Considering the relative reaction rates of different alkyl bromides with ethoxide ion, we observe a clear pattern where larger alkyl groups significantly reduce the reaction rate. This decline is attributable to increased steric hindrance: larger groups, such as the neopentyl group in neopentyl bromide, create a more congested environment around the reactive carbon-halogen bond, making it more difficult for the nucleophile (ethoxide ion) to reach and react with the carbon atom.

Nucleophilic Substitution Reactions

Nucleophilic substitution reactions are a cornerstone of organic chemistry. These reactions involve the exchange of a nucleophile—a molecule or ion that donates an electron pair to form a bond—with a leaving group attached to a carbon atom. One of the most common types is called the bimolecular nucleophilic substitution, or SN2 reaction, where the bond formation and bond breaking occur simultaneously.

Role of the Alkyl Group in SN2 Reactions

In the SN2 mechanism, a nucleophile attacks from the opposite direction to the leaving group, resulting in an inversion of stereochemistry at the reaction center. The size of the alkyl group (G) attached to the carbon bearing the leaving group plays a significant role. Smaller groups, like in methyl bromide, allow for easier access to the carbon atom, making the reaction more likely to occur quickly. Larger groups slow down the reaction because they create a shield around the reactive center, which hinders the nucleophile's access.