For each of the following reactions, give the substitution...

For each of the following reactions, give the substitution products; if the products can exist as stereoisomers, show what stereoisomers are obtained: a. $(R)$-2-bromopentane + high concentration of $\mathrm{CH}_{3} \mathrm{O}^{-}$ d. $(R)-3$-bromo-3-methylhexane $+\mathrm{CH}_{3} \mathrm{OH}$ b. trans-1-bromo-4-methylcyclohexane $+$ high concentration of $\mathrm{CH}_{3} \mathrm{O}^{-} \quad$ e. 1-bromo-1-methylcyclohexane $+\mathrm{CH}_{3} \mathrm{OH}$ c. 3-bromo-3-methylpentane $+\mathrm{CH}_{3} \mathrm{OH}$

For each of the following reactions, give the substitution products; if the products can exist as stereoisomers, show what stereoisomers are obtained:
a. $(R)$-2-bromopentane + high concentration of $\mathrm{CH}_{3} \mathrm{O}^{-}$
d. $(R)-3$-bromo-3-methylhexane $+\mathrm{CH}_{3} \mathrm{OH}$
b. trans-1-bromo-4-methylcyclohexane $+$ high concentration of $\mathrm{CH}_{3} \mathrm{O}^{-} \quad$ e. 1-bromo-1-methylcyclohexane $+\mathrm{CH}_{3} \mathrm{OH}$
c. 3-bromo-3-methylpentane $+\mathrm{CH}_{3} \mathrm{OH}$

Key Concepts

Nucleophilic Substitution

This concept describes reactions in which a nucleophile displaces a leaving group attached to a carbon atom. The substitution can occur via different pathways depending on the structure of the organic substrate and the reaction conditions, leading to variations in stereochemical outcomes.

SN2 Mechanism

In an SN2 reaction, the nucleophile attacks the substrate in a single, concerted step from the side opposite to the leaving group. This backside attack results in a complete inversion of stereochemistry at the carbon center where the substitution occurs. The SN2 mechanism is typically favored by primary and some secondary substrates and by conditions with high concentrations of strong nucleophiles.

SN1 Mechanism

The SN1 mechanism involves a two-step process where the leaving group first departs to form a carbocation intermediate. The planar nature of the sp2-hybridized carbocation allows the nucleophile to attack from either side, often resulting in a mixture of products with different stereochemistry (commonly leading to racemization). This mechanism is generally favored by tertiary substrates where the carbocation can be stabilized by surrounding groups.

Stereochemical Outcomes in Substitution Reactions

Understanding the stereochemical implications of substitution reactions is crucial. The SN2 pathway leads to inversion of configuration at the reactive center, while the SN1 pathway, due to the formation of a planar intermediate, typically produces racemic or a mixture of stereoisomers. These outcomes are essential for predicting the properties and reactivity of the products.

Substrate Structure and Reaction Pathway

The nature of the substrate—whether primary, secondary, or tertiary—greatly influences the reaction mechanism chosen by a substitution reaction. Primary substrates usually favor the SN2 pathway providing pronounced stereochemical inversion, whereas tertiary substrates tend to undergo SN1 reactions due to the formation of stable carbocations, leading to diminished stereochemical control.

Transcript

00:01 This question asked for the products of each of these reactions, including stereochemistry, and we are only drawing substitution products here.

00:09 So starting with letter a, we have a secondary leaving group and a good nucleophile.

00:14 So this will be an sn2 reaction, which means we'll only have one product, because sn2 reactions give us complete inversion of stereochemistry.

00:23 So we'll put the methoxy on a dash, since the bromine was on a wedge here, and the hydrogen will be on a wedge now.

00:32 So there's our single product for part a.

00:36 For part b, we have r3 -bromote -3 methyl -heptane and methanol.

00:41 So now we have a tertiary leaving group and a weak nucleophile.

00:45 So this will be sn1 for substitution.

00:48 So we'll get two anantiomers here because if we're doing sn1, it goes through a carbocadion, so you can end up with the nucleophile on either side.

00:58 So we'll have one where it's on the same side that the bromine was on, that will look like this, and we'll have one where it's on the opposite side like this.

01:12 So there's our two stereo ismeris for part b for part c.

01:17 This is also an sn1 reaction because we have a primary benzylic carbocation there and a weak nucleophile.

01:24 There is no stereochemistry here because we have two hydrogens on that carbon, so it's not a stereo center.

01:30 So we'll just get one product that looks like this.

01:33 We'll just replace the chlorine with the athoxy group...

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