• polar aprotic
• stabilze (+) which allows the nucleophile to attack
• need good nucleophile
• ex: DMSO, DMF, HMPA, acetonic

What are some characteristics of S_N 1 reactions as polar solvents?

• polar protic
• stabilizes (+) and (-)
• can separate charges (?)
• ex: H₂O and alcohol

• strong branched base and conjugate acid
• polar protic and aprotic (removes B-H)
• ex: CH₃O ^- in CH₃O/DMSO

What are some characteristics of E1 reactions as polar solvents?

• polar protic
• stabilize (+) and (-)
• solvolysis
• competes with S_N1
• ex: H₂O and alcohol

higher.

because:

charged species > lone pair e^- > H₂O or alcohol

• polar protic: S_N1 and E1 reactions
• H-bonding donors and acceptors
• stabilize carbocation and anion
• acids R-X ionization or solvolysis
• ex: H₂O and alcohol

What are some factors of polar aprotic solvents in terms of nucleophilicity?

• polar aprotic: S_N2, E2
• little or no h-bonding
• stabilizes (+) so nucleophile can attack
• ex: acetone, DMSO, DMF, HMPA

lower.

because

CH₃^- > H₂N^- > RO ^- > F^- > Cl^- > Br^- > I^-

pKa values decrease -->

In polar protic: nucleophilicity & basicity both decrease

In polar aprotic: nuclephilicity increases and basicity decreases

nucleophilicity: 1^o > 2^o > 3^o

basicity:  3^o > 2^o > 1^o

related to the stability of an atom; the best leaving group is a weak base=conjugate base of a strong acid

I^- > Br^- > Cl^- > > F ^-

1^o > 2^o > > > (3^o)

2 important factors: steric factor and branching

Both slow the reaction to allow for a backside attack!

3^o > 2^o > 1^o

must have beta-hydrogens and anti-configuration is preferred

3^o > 2^o > > > (1^o)

Electronic factor is important to stabilize carbozation

• bimolecular
• Rate = k[Rx][nu]
• back side attack! Me>1>2>>3
• TS (no intermediate)one step
• need a good nucleophile (SH-, -OCH3, -OH, -I)
• need a good LG
• iversion -->stereochemistry
• polar aprotic solvent
• E2 product possible

• bimolecular
• Rate = k[Rx][base]
• TS concerted
• B-H's best if anti (or at least coplanar)
• strong branched base helps
• Zaitsen's Rule = most branched alkenes preferred
• sometimes competes with S_N2

• unimolecular
• rate = k[R-X]
• carbocation intermediate ( 3 > 2 >>>1)
• branching desirable
• 2-step reaction
• racemic products/rearrangements possible (look at 2^o)
• ok nucleophile (H2O, H2S, CH2OH, Cl-)
• but need good LG
• polar protic solvent; solvolysis stabilizes (+) and (-)
• E1 products are probable

• unimolecular
• carbocation intermediate (3>2>>>1)
• branching helps
• rearrangements pssible
• Zaitsen's rule (most substituted alkene)
• polar protic solvent
• solvolysis (no strong base needed)
• competes with S_N1

What happens when a methyl halide or a primary alkyl halide reacts with a Lewis base?

Na⁺CH₃CH₂O^- + Br-CH₂CH₃--?

The Lewis base replaces the halogen, which gets expelled as halide ion

---> CH₃CH₂O-CH₂CH₃ + Na⁺ Br^-

A Lewis base acting as a nucleophile donates an electron pair to an eletrophile to displace a leaving group.

Or in the words of Ganem: something goes in, something gets kicked out.

CH₃O^- + CH₃-Cl -----> CH₃-O-CH₃ + Cl^-

alpha-carbons: the carbon bearing the halogen (in alkyl halides)

beta-carbon: the carbons adjacent to the alpha-carbon

primary: substitution

tertiary: β-elimination

both double: rate = k[A][B]

only A doubles=k[A]

the overall kinetic order for a reaction is the sum of the powers of all the concentrations in the rate law.

the kinetic order in each reactant is the power to which its concentration is raised in the rate law.

S=substitution

N=nucleophilic

2=bimolecular

1. the concentration terms of the rate law show what atoms are involved in the rate-limiting step
2. mechanisms that are not constitent with the rate law are ruled out
3. the simplest mechanisms consistent with the rate law is used

what three different ways can a substitution reaction occur at the stereocenter

• with rentention of configuration at the stereocenter
• with the inversion of configuration at the stereocenter
• with a combination of the 2 (inversion and retention)

inversion of configuration is usually observed in S_N2 reactions at carbon stereocenters.

any effect on a chemical phenomenom, such as a reaction, caused by van der Waals repulsions

S_N2 reactions of branched alkyl halides are retarded by a steric effect

secondary and teriary alkyl halides react so slowly in S_N2 reactions that the rates of elimination raectionscompete with the rates of substitution.

the rates of the S_N2 of teritary alkyl halides are so slow that elimination is the only reaction observed.

whether the solvent is protic, has the most effect on rate.

 the change of solvent will have a great effect on the rate of its S_N2 reactions depending on the strength of the ion

syn- the dihedral angle between the C-H and C-X bond is 0°  (the H and V groups leave from the same side of the reference plane)

anti- the dihedral angle between the C-H and C-X bonds is 180° (H and X groups leave from opposite sides of the reference plane

1. its transition state has a staggered conformation, which is more stable than syn-, therefore anti- is faster
2. the base and the leaving group are on opposite sides of the molecule
3. more favorable because all electron displacements are backside

nucleophile: S_N2 reaction

Bronsted base: E2 reaction

1. the structure of the alkyl halide (at both the alpha and beta carbons)
2. the structure of the base

1. is the alkyl halide primary, secondary, or tertiary
2. is a lewis base present (if yes, is it a good nucleophile, strong Bronsted base, or both?)
3. what is the solvent? (polar protic, polar aprotic, or mixtures of both?)