SN2 LAB 1. What determines a g

1)Because the nucleophile is involved in the rate-determiningstep of SN2 reactions, stronger nucleophiles react faster. Strongernucleophiles are said to have increased nucleophilicity. In the gasphase, there is a correlation between increased relativenucleophilicity and increased base strength, although there aremany exceptions to this trend in solution (see below). In general,within a period of the periodic table, nucleophilicity increasesfrom right to left. Furthermore, for different reagents with thesame nucleophilic atom, an anion is a better nucleophile than aneutral species.The basicity of a nucleophile is important when youwant to favor SN2 on a hindered alkylhalide, like a secondary alkylhalide. Some good nucleophiles are strong bases, and some areweakbases. Base strength is measured by looking at the pKaof theconjugate acid. A weak basewill have a conjugate acid with apKaless than about 8.a)nucleophile is positive charge loving species..when the negativecharge is more in a nucleophile they can attract positve chargespecies more strongly.(. Increasing negative charge increasesnucleophilicity. Water and methanol are badnucleophiles, but if you deprotonate them, they become goodnucleophiles.b) nucleophile needs to be small enough to fit through the othergroups to be able to attack if it cant reach it cantsubstitute.c)The basicity of a nucleophile is important when you want to favorSN2 on a hindered alkylhalide, like a secondary alkyl halide. Somegood nucleophiles are strong bases, and some areweak bases. Basestrength is measured by looking at the pKaof the conjugate acid. Aweak basewill have a conjugate acid with a pKaless than about8.

2)The Nature of the Leaving GroupIn order to understand the nature of the leaving group, it isimportant to first discuss factors that help determine whether aspecies will be a strong base or weak base. If you remember fromgeneral chemistry, a Lewis base is defined as a species thatdonates a pair of electrons to form a covalent bond. The factorsthat will determine whether a species wants to share its electronsor 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 theperiodic table as shown in the diagram below, electronegativityincreases. As electronegativity increases, basicity will decrease,meaning a species will be less likely to act as base; that is, thespecies will be less likely to share its electrons.As Size Increases, Basicity Decreases: In general, if we move fromthe top of the periodic table to the bottom of the periodic tableas shown in the diagram below, the size of an atom will increase.As size increases, basicity will decrease, meaning a species willbe less likely to act as a base; that is, the species will be lesslikely to share its electrons.Resonance Decreases Basicity: The third factor to consider indetermining whether or not a species will be a strong or weak baseis resonance. As you may remember from general chemistry, theformation of a resonance stabilized structure results in a speciesthat is less willing to share its electrons. Since strong bases, bydefinition, want to share their electrons, resonance stabilizedstructures are weak bases.

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

3)The “big barrier” to the SN2 reaction is steric hindrance. Therate of SN2 reactions goes primary alkyl halide > secondaryalkylhalide> tertiary alkyl halideIf the substrate is tertiary, we can rule out SN2, because tertiarycarbons are very sterically hindered.solvent should be polaraprotic(DMSO,ACETONE)1) Carbocations are stabilized by neighboring carbon atoms.2) Carbocations are stabilized by neighboring carbon-carbonmultiple bonds.3) Carbocations are stabilized by adjacent lone pairs.

4)The reaction most often occurs at an aliphatic sp3 carboncenter with an electronegative, stable leaving group attached to it(often denoted X), which is frequently a halide atom. The breakingof the C–X bond and the formation of the new bond (often denotedC–Y or C–Nu) occur simultaneously through a transition state inwhich a carbon under nucleophilic attack is pentacoordinate, andapproximately sp2 hybridised. The nucleophile attacks the carbon at180° to the leaving group, since this provides the best overlapbetween the nucleophile’s lone pair and the C–X σ* antibondingorbital. The leaving group is then pushed off the opposite side andthe product is formed with inversion of the tetrahedral geometry atthe central atom.If the substrate under nucleophilic attack is chiral, this oftenleads to inversion of configuration (stereochemistry), called aWalden inversion.In an example of the SN2 reaction, the attack of Br− (thenucleophile) on an ethyl chloride (the electrophile) results inethyl bromide, with chloride ejected as the leaving group.N2 attackoccurs if the backside route of attack is not sterically hinderedby substituents on the substrate. Therefore, this mechanism usuallyoccurs at unhindered primary and secondary carbon centres. If thereis steric crowding on the substrate near the leaving group, such asat a tertiary carbon centre, the substitution will involve an SN1rather than an SN2 mechanism, (an SN1 would also be more likely inthis case because a sufficiently stable carbocation intermediarycould be formed).5)Bimolecular: A bimolecular reaction is onewhose rate depends on the concentrations oftwo of its reactants.● SN2 reactions happen in one step – the nucleophile attacks thesubstrate as theleaving group leaves the substrate.Recall that the rate of a reaction depends on theslowest step. Inbimolecular reactions, therefore, the slowstep involves tworeactants. For SN2 reactions, there are only two reactants; thismeans that the slow step is the only step.SN1 reactions are nucleophilic substitutions, involving anucleophile replacing a leaving group (just like SN2).However: SN1 reactions are unimolecular: the rate of this reactiondepends only on the concentration of one reactant.● SN1 reactions happen in two steps:1. The leaving group leaves, and the substrate forms acarbocationintermediate.2. The nucleophile attacks the carbocation, forming theproduct.