Question

SN2 LAB

1. What determines a good nucleophile for S_N2?

a) Consider the role that electron density plays in the strength of a nucleophile.

b) Consider the attacking ability purely through steric means.

c) Consider the correlation of acidity/basicity to nucleophilicity.

What determines a good leaving group S_N2?

a) Consider the role of acid-base chemistry.

b) Consider modifications that might make a poor leaving group into a better one.

c) Consider any properties that might make something both a good leaving group as well as a good nucleophile.

3. What properties of a substrate make it ideal for doing S_N2 substitution?

a) Consider effects that might stabilize a carbocation.

b) Consider the role of solvents.

4. What are the stereochemical implications for bimolecular (S_N2) substitutions?

List the stereochemical implications for bimolecular (S_N2) substitutions?

5. To what aspects of substitution reactions are the terms unimolecular, and bimolecular referring?

To what aspects of substitution reactions are the terms unimolecular, and bimolecularreferring?

Answer 1

1)Because the nucleophile is involved in the rate-determining step of SN2 reactions, stronger nucleophiles react faster. Stronger nucleophiles are said to have increased nucleophilicity. In the gas phase, there is a correlation between increased relative nucleophilicity and increased base strength, although there are many exceptions to this trend in solution (see below). In general, within a period of the periodic table, nucleophilicity increases from right to left. Furthermore, for different reagents with the same nucleophilic atom, an anion is a better nucleophile than a neutral species.The basicity of a nucleophile is important when you want to favor SN2 on a hindered alkylhalide, like a secondary alkyl halide. Some good nucleophiles are strong bases, and some areweak bases. Base strength is measured by looking at the pKaof the conjugate acid. A weak basewill have a conjugate acid with a pKaless than about 8.
a)nucleophile is positive charge loving species..when the negative charge is more in a nucleophile they can attract positve charge species more strongly.(. Increasing negative charge increases nucleophilicity. Water and methanol are bad
nucleophiles, but if you deprotonate them, they become good nucleophiles.
b) nucleophile needs to be small enough to fit through the other groups to be able to attack if it cant reach it cant substitute.
c)The basicity of a nucleophile is important when you want to favor SN2 on a hindered alkylhalide, like a secondary alkyl halide. Some good nucleophiles are strong bases, and some areweak bases. Base strength is measured by looking at the pKaof the conjugate acid. A weak basewill have a conjugate acid with a pKaless than about 8.

2)The Nature of the Leaving Group
In order to understand the nature of the leaving group, it is important to first discuss factors that help determine whether a species will be a strong base or weak base. If you remember from general chemistry, a Lewis base is defined as a species that donates a pair of electrons to form a covalent bond. The factors that will determine whether a species wants to share its electrons or not include electronegativity, size, and resonance.

As Electronegativity Increases, Basicity Decreases: In general, if we move from the left of the periodic table to the right of the periodic table as shown in the diagram below, electronegativity increases. As electronegativity increases, basicity will decrease, meaning a species will be less likely to act as base; that is, the species will be less likely to share its electrons.
As Size Increases, Basicity Decreases: In general, if we move from the top of the periodic table to the bottom of the periodic table as shown in the diagram below, the size of an atom will increase. As size increases, basicity will decrease, meaning a species will be less likely to act as a base; that is, the species will be less likely to share its electrons.
Resonance Decreases Basicity: The third factor to consider in determining whether or not a species will be a strong or weak base is resonance. As you may remember from general chemistry, the formation of a resonance stabilized structure results in a species that is less willing to share its electrons. Since strong bases, by definition, want to share their electrons, resonance stabilized structures are weak bases.

Weak Bases are the Best Leaving Groups
Now that we understand how electronegativity, size, and resonance affect basicity, we can combine these concepts with the fact that weak bases make the best leaving groups. Think about why this might be true. In order for a leaving group to leave, it must be able to accept electrons. A strong bases wants to donate electrons; therefore, the leaving group must be a weak base. We will now revisit electronegativity, size, and resonance, moving our focus to the leaving group, as well providing actual examples.

3)The “big barrier” to the SN2 reaction is steric hindrance. The rate of SN2 reactions goes primary alkyl halide > secondary alkylhalide> tertiary alkyl halide
If the substrate is tertiary, we can rule out SN2, because tertiary carbons are very sterically hindered.solvent should be polar aprotic(DMSO,ACETONE)
1) Carbocations are stabilized by neighboring carbon atoms.
2) Carbocations are stabilized by neighboring carbon-carbon multiple bonds.
3) Carbocations are stabilized by adjacent lone pairs.

4)The reaction most often occurs at an aliphatic sp3 carbon center with an electronegative, stable leaving group attached to it (often denoted X), which is frequently a halide atom. The breaking of the C–X bond and the formation of the new bond (often denoted C–Y or C–Nu) occur simultaneously through a transition state in which a carbon under nucleophilic attack is pentacoordinate, and approximately sp2 hybridised. The nucleophile attacks the carbon at 180° to the leaving group, since this provides the best overlap between the nucleophile's lone pair and the C–X σ* antibonding orbital. The leaving group is then pushed off the opposite side and the product is formed with inversion of the tetrahedral geometry at the central atom.
If the substrate under nucleophilic attack is chiral, this often leads to inversion of configuration (stereochemistry), called a Walden inversion.
In an example of the SN2 reaction, the attack of Br− (the nucleophile) on an ethyl chloride (the electrophile) results in ethyl bromide, with chloride ejected as the leaving group.N2 attack occurs if the backside route of attack is not sterically hindered by substituents on the substrate. Therefore, this mechanism usually occurs at unhindered primary and secondary carbon centres. If there is steric crowding on the substrate near the leaving group, such as at a tertiary carbon centre, the substitution will involve an SN1 rather than an SN2 mechanism, (an SN1 would also be more likely in this case because a sufficiently stable carbocation intermediary could be formed).
5)Bimolecular: A bimolecular reaction is one
whose rate depends on the concentrations of
two of its reactants.
● SN2 reactions happen in one step – the nucleophile attacks the substrate as theleaving group leaves the substrate.
Recall that the rate of a reaction depends on theslowest step. In bimolecular reactions, therefore, the slowstep involves two reactants. For SN2 reactions, there are only two reactants; this means that the slow step is the only step.
SN1 reactions are nucleophilic substitutions, involving a
nucleophile replacing a leaving group (just like SN2).
However: SN1 reactions are unimolecular: the rate of this reaction depends only on the concentration of one reactant.
● SN1 reactions happen in two steps:
1. The leaving group leaves, and the substrate forms acarbocation intermediate.
2. The nucleophile attacks the carbocation, forming theproduct.