SN2

If a halogen replaces a hydrogen, you get a …. (A)

haloalkane also known as alkyl halides

what are the steps to naming a haloalkane?

• Find the parent alkane (the one closest to the halogen)

• Number the chain so the halogen gets the lowest number

• Add the halogen prefix + number before the alkane name

?

name this

1S,2R)-1-Bromo-2-

fluorocyclohexane

name this

2-Iodo-2-

methylpentane

What is a C–X bond?

Back: A bond between carbon and a halogen (F, Cl, Br, or I).

Front: Why is the C–X bond polarized?

Back: Because halogens are more electronegative and pull electrons toward themselves.

Front: In a C–X bond, which atom is δ⁺ and which is δ⁻?

Back: Carbon is δ⁺, halogen is δ⁻.

Front: Which way does the dipole arrow point in a C–X bond?

Back: Toward the halogen.

Front: In alkyl halides, where do nucleophiles attack?

Back: The carbon attached to the halogen.

Front: What does R–X represent?

Back: An alkyl halide (a carbon chain bonded to a halogen).

Front: How does C–X bond length change from F → I?

Back: It increases (gets longer).

Front: How does C–X bond strength change from F → I?

Back: It decreases (gets weaker).

Front: How do boiling points change from R–F to R–I?

Back: They increase.

Front: What causes boiling points of R–X to increase down the group?

Back: Increased size and polarizability → stronger London dispersion forces.

Front: What is a nucleophile?

Back: A molecule or ion that donates a pair of electrons (Lewis base).

what is this? and explain it?

Back: A reaction where a nucleophile replaces a leaving group on a carbon.

Front: Why do nucleophiles react?

Back: They have extra electrons and want to donate them.

Front: Where does nucleophilic substitution happen?

Back: At an sp³ (tetrahedral) carbon.

draw the arrows

Front: Where does the first curved arrow start and end?

-Back: Starts at the lone pair on Nu: and points to the carbon.

Front: Where is the leaving group before the reaction?

-Back: Directly bonded to the carbon (written as C–Lv).

Front: Where does the second curved arrow start and end?

-Back: Starts at the C–Lv bond and points to Lv.

draw the arrows (DONT ERASE)

Front: Where does the FIRST curved arrow start and end?

Back: Starts at the lone pair on Nu: and points to the carbon of CH₃Br.

Front: Where does the SECOND curved arrow start and end?

Back: Starts at the C–Br bond and points to Br.

use this to draw all the finish all the reactions

(look at the question before)

Front: In SN2, when does bond formation occur relative to bond breaking?

Back: At the same time (simultaneously).

Front: What does the “2” in SN2 stand for?

Back: Bimolecular (two species involved in the rate-determining step).

Front: Which two species are involved in SN2?

Back: The nucleophile and the substrate (R–X).

Front: Does SN2 have an intermediate?

No

Show the mechanism

• Rate = k[CH₃Br][OH⁻]

• Na⁺ is not in the rate law → spectator

draw the reaction coordinate diagram for this reaction

Front: What does the top of the hill represent?

Back: The transition state.

Front: What bonds are changing at the SN2 transition state?

Back: C–Nu forming and C–Lv breaking at the same time.

Front: What does ΔH measure?

Back: Energy difference between products and reactants.

Front: If products are lower in energy than reactants, what is ΔH?

Back: Negative (exothermic).

Front: In an endergonic reaction, are products higher or lower in energy?

Higher

draw the inversion of the chiral center

draw the sn2 reaction mechanism

Front: What causes inversion in SN2?

Back: Backside attack of the nucleophile.

Front: What happens to the 3D arrangement at the carbon after SN2?

it flips/inverts

Front: What ALWAYS inverts in SN2?

Back: The relative configuration at the carbon.

Front: Does SN1 also give inversion?

Back: No (SN1 gives racemization).

Front: Why is inversion unavoidable in SN2?

Back: The nucleophile must attack from the backside as the leaving group leaves.

Front: What makes a good leaving group?

Back: A stable anion.

Front: Good leaving groups are conjugate bases of what?

Strong acids

Front: Rank halides by leaving group ability.

Back: I⁻ > Br⁻ > Cl⁻ > F⁻.

Front: Why is F⁻ a poor leaving group?

Back: It is small and unstable as an anion.

Front: Are strong bases good leaving groups?

NO

are these good leaving groups?

no they rarely function

They are ordered from best leaving group → worst leaving group.

Front: Why is TsCl used on alcohols?

Back: To convert OH into a good leaving group (OTs).

Front: Is OTs⁻ a strong or weak base?

Back: Very weak base.

Front: What does “steric hindrance” mean?

Back: Physical crowding that blocks nucleophile approach.

Front: Rank substrates by SN2 reactivity.

Back: Methyl > Primary > Secondary >> Tertiary.

Front: Which substrate does NOT undergo SN2?

Back: Tertiary.

Front: Why is SN2 negligible for tertiary substrates?

Back: Too much steric hindrance blocks backside attack.

Front: What is β-branching?

Back: Alkyl substitution on the carbon adjacent to the reacting carbon.

Front: Which carbon is the β-carbon?

Back: The carbon next to the one bonded to the leaving group.

Front: How does β-branching affect SN2?

Back: It slows SN2 by blocking backside attack.

Front: What happens to SN2 rate as β-branching increases?

Back: It decreases dramatically.

Front: Why is neopentyl bromide bad for SN2?

Back: Three β-branches cause extreme steric hindrance.

Front: What is a protic solvent?

Back: A solvent that can donate a hydrogen bond.

Front: What structural feature makes a solvent protic?

Back: An H bonded to O or N.

what are these?

protic solvents

Front: Why are aprotic solvents not hydrogen-bond donors?

Back: They have no H bonded to O or N.

Front: Can a solvent be polar and aprotic?

Yes

• The molecule contains electronegative atoms (like O, N, S) → polarity

• All hydrogens are bonded to carbon (or no hydrogens at all) → aprotic

what are these?

aprotic solvents

what does solvation mean?

• Solute = the thing you dissolve

• Solvent = the liquid doing the dissolving

Solvation is when the solvent wraps around the solute and stabilizes it.

positive ions → solvent points its positive or negative end at them

negative ions → solvent points its positive or negative end at them

• Positive ions → solvent points its negative end at them

• Negative ions → solvent points its positive end at them

Solvation can slow down or weaken nucleophiles, depending on the solvent. how?

• Polar protic solvents strongly solvate (hug) nucleophiles → nucleophile weaker

• Polar aprotic solvents weakly solvate → nucleophile stronger

Front: What must a nucleophile do before attacking in SN2?

Back: Escape its solvation shell.

Front: What does strong solvation do to SN2 rate?

Back: Slows it down.

Front: Why does weak solvation speed up SN2?

Back: Less energy is needed to free the nucleophile.

Front: How is negative charge distributed in the SN2 transition state? draw it!

Back: Spread between the nucleophile and leaving group.

Front: Do protic solvents solvate nucleophiles strongly or weakly?

Back: Strongly — protic solvents tightly surround nucleophiles with hydrogen bonding.

Front: Do polar aprotic solvents strongly or weakly solvate nucleophiles?

Back: Weakly — they do not hydrogen-bond to nucleophiles.

Front: Which solvent type gives faster SN2: protic or polar aprotic?

Back: Polar aprotic.

draw it

classify these

Front: What is nucleophilicity?

Back: A kinetic property measuring how fast a nucleophile attacks carbon.

Front: What is basicity?

Back: A thermodynamic property measuring how strongly a base binds H⁺.

Front: Which property is measured by reaction rate?

Back: Nucleophilicity.

Front: Which property is measured by equilibrium position?

Basicity

Front: What does a lower activation barrier mean for nucleophilicity?

Back: Faster reaction → stronger nucleophile.

Front: In protic solvent, are larger or smaller nucleophiles better?

Back: Larger.

Front: Rank halides by nucleophilicity in protic solvent.

Back: I⁻ > Br⁻ > Cl⁻ > F⁻.

Front: Are neutral molecules good nucleophiles in protic solvent?

Back: No.

Front: Within a period, which direction increases nucleophilicity?

Right to L

Across a period, nucleophilicity follows basicity.

1⃣ Atoms get less electronegative

• They hold electrons less tightly

• Lone pairs are more available to attack

Example:

• O is more electronegative than N

• N holds electrons less tightly → better attacker

Front: Why does nucleophilicity increase right to left in a row?

Back: Electrons are held less tightly.

Front: Which is the best nucleophile in this set: F⁻, OH⁻, NH₂⁻, CH₃⁻?

Back: CH₃⁻.

Which is the best nucleophile?
F⁻, OH⁻, NH₂⁻, CH₃⁻

All of these are:

• Negatively charged

• Same period (row 2)

That tells you exactly which rule to use.

Front: Given the same nucleophilic atom, which is more nucleophilic: neutral or anion?

Anion

Front: When the nucleophilic atom is the same, what controls nucleophilicity?

Back: Basicity.

Front: Rank these by nucleophilicity: RCOO⁻, HO⁻, RO⁻

Back: RCOO⁻ < HO⁻ < RO⁻

Acetate is a:

• Good leaving group

• Weak base

But:

Good leaving group ≠ good nucleophile

Resonance makes things stable → stable things don’t attack.

Front: How does conjugate-acid pKa relate to nucleophilicity?

Back: Higher pKa → stronger base → better nucleophile.

Front: For neutral nucleophiles, how does atomic size affect nucleophilicity?

Back: Larger atom = better nucleophile.

Front: How does bulkiness affect nucleophilicity?

Back: More bulk = lower nucleophilicity.

Front: Which is a better nucleophile: ethoxide or tert-butoxide?

Back: Ethoxide.

Q: Best halide nucleophile in polar protic solvent?

A: I⁻

Q: Best halide nucleophile in polar aprotic solvent?

F^-1

Q: Why is F⁻ weak in protic solvents?

A: Strong hydrogen-bonding → heavy solvation

Q: Why is F⁻ strong in aprotic solvents?

A: Poorly solvated + high basicity

Q: One rule to remember solvent effects?

A: Protic → size matters; aprotic → basicity matters

Why the drawings get “bigger” going down a row?

As X gets larger (F → Cl → Br → I):

• X orbitals are bigger

• σ and σ* orbitals become more polarized

• More electron density shifts toward X

Which orbital stabilizes a bond: σ or σ*?

σ stabilizes, σ* destabilizes.