Organic Chemistry 1 Exam 3

Terminal alkene

End of chain and monosubstituted

Internal alkene

Within the chain and disubstituted

Acetylene

Triple bond with just H on both sides

Reagents for Alkylation of Terminal Alkenes

NaNH2 and CH3CH2X

What are the steps of alkylation of terminal alkenes?

1. The lone pair on NH2 takes the H and the H leaves its e- behind

2. LP on C goes to the partially positive part (C) of the electrophile (dipole moment with C and X)

What is the goal of alkylation of terminal alkynes?

To deprotonate the alkyne and extend the carbon chain

What are the electrophiles types you can have for alkylation of terminal alkenes?

Primary RX or Methyl Halide

Alkynes have higher priority than what functional groups

Halogens or ethers

When a molecule has an alkyne and alkene, which one takes priority

Neither, you just do the lowest numbering possible. If there is a tie, then choose alkenes for lowest numbering.

What is the goal of dehydrohalogenation (using dihalides)

Alkene to Alkyne

What does a dehydrohalogenation reaction do?

Removes hydrogen and halogen

What is a vicinal halide?

Halides are next door to each other

What is a geminal halide?

On the same carbon

What are the steps of a dehydrohalogenation reaction?

1. Anti-addition of halides to both sides of DB

2. If halide of interest is on C2, pull H off of C3 using LP on NH2

   1. LP on N takes H

   2. the H e-s go between C2 and C3 to make a DB

   3. e’s on the halide leave

3. Use NH2 to take off other H and other halide e’s move too

4. If the product still has an acidic H, use NaNH2 again

   1. Pull off the H, now its a carboanion, so use H2O

How to do dehydrohalogenation with a vinyl halide?

Only have to use 1 eq. to get Triple Bond

What reaction to get from alkyne to alkene to alkane?

Hydrohalogenation

What reagents do you use for hydrohalogentation?

Alkyne and Dihalide

What are the steps of a hydrohalogenation reaction?

1. Halogen LP takes 1 bond, the other bond takes the halogen

2. Bridged intermediate forms (to positive halide). Negative halide (left from last step) LP takes other bond and halides form on either side of the DB

3. Still a pi bond left so the dihalide reacts again with it

4. Now there’s another bridge, so add the negative halide from the last time to it

What is the product of a hydrohalogenation reaction?

An Alkane with 2 Halides on each side

What does 1 eq. do in hydrohalogenation

1 X to each side

What does 2 eq. do in hydrohalogenation

2 halides to each side

What is the goal of addition of hydrogen halide?

2 halides on ONE C (alkyne to alkane if 2 eq.)

Hydrogen Halide steps

1. One bond takes H and X gets those e’s

   1. A carbocation is formed but no shifts

2. Add negative X to carbocation

3. Since there is still a DB, add H-X again then add X- to carbocation

Hydroboration Oxidation reagent for internal alkenes

BH3

Hydroboration Oxidation reagent for terminal alkenes

(sia)2BH

Hydroboration Oxidation Steps

1. one bond takes the H, and second bond takes the B

   1. So H on one side and B(sia)2 on other side of DB

2. Replace B(sia)s with OH (no work shown)

3. An enol is produced which goes through tautomerism (DB is moved to the O)

Tautomerism steps

1. OH- takes H, H leaves its e’s and O on molecule is negative

2. LP on O makes DB and current DB makes carboanion

3. Use H2O to remove carboanion

What are the reagents for acid-catalyzed hydration?

HgSO4, heat

Steps of Acid-Catalyzed Hydration

1. One bond takes Hg2+

2. LP on Hg+ goes to C+

   1. Hg 2+ bridge is formed and water breaks it up

3. Get rid of O+ with more water

4. Hg heads off, leaves its e- behind to form DB and the previous DB goes to O

5. Then Acid-Catalyzed tautomerism with enol

   1. LP from OH makes DB, and previous DB takes H

   2. Water takes H from o+ and just left with O and DB

Catalytic Reduction from alkyne to alkane

Use H2 and catalyse (Pt, Ni, Pd)

• Syn addition of protons to both sides

Catalytic Reduction from alkyne to alkene

Use H2 and Lindlar’s Catayst (cis alkene product)

What does a catalytic reduction using H2 and lindar’s Catalyse yield?

A cis alkene

Hydroboration-Protonolysis reagent

(sia)2BH

Hydroboration-protonolysis steps

1. Hydroboration: H on less sub side & B(sia)2 on more sub side

2. Just switch B(sia)2 for H so there’s nothing

Dissolving-Metal Reduction includes what important thing

Sodium radical as reagent

Dissolving-Metal Reduction steps

1. One bond gives radical and Na gives radical

2. left with a radical C and carbocation on each side of DB

   1. Fix the CARBOCATION first by using H-NH2 and grabbing an H

3. Use Na radical to add e- to C radical to create carcation

4. Fix carbocation by using N-NH2 again

To get an alkene from an alkyne, you must do what type of reaction

Catalytic reduction

To get a cis alkene from an alkyne, what reactions can you do

• Hydroboration Oxidation

• Hydroboration Protonolysis

To get a trans alkene from alkyne, what reaction

Dissolving metal reduction

Alkyne Hydration, what 2 options do you have to use

Markovnikov: Use HgSO4

Anti-Markovnikov: Hydroboration oxidation

You can get a cis alkene from an alkyne by using

Hydroboration proteolysis

Bromo-benzene

on benzene ring

Benzylic bromine

Br is one C away from the ring

What is radical halogenation of Alkanes?

H is replaced by halide

What are the steps and explanations of radical chain medhanism?

1. Initiation: Use a dihalide to create the radical (need heat or light as bottom reagent)

2. Propagation: Continue/pass on radical (2 steps)

   1. ex. break C-H and then replace with Cl

3. Termination: Kill the radical (can use another radical and create a dihalide)

Bromination preference

tertiary

Bromine should always add to the

more substitued side

Chlorination doesn’t really prefer

tertiary position

For chlorination, you have to

calcualte the relative reactivity of each position to determine the product

How do you calculate the relative reactivity of each posiiton in Chlorinatoin?

You draw in all of the H’s and replace one at a time with halide (watch out for duplicates)

Why is bromination more selective than chlorination

Because of Hammond’s postulate

What is does hammond’s postulate say

In exo rxns, TS is closer to reactions

In endo rxns, TS is closer to products

Chlorination is what kind of reaction and how does this relate to Hammond’s postulate?

Chlorination is an exothermic rxn, so changing radical stability doesn’t change TS energy

Bromination is what kind of reaction and how does this relate to Hammond’s postulate?

Bromination is an endothermic reaction so changing stability does change TS energy

For bromination, which has more energy, a primary or tertiary position?

A primary position

What is special about allylic halogenation?

In the 2nd step of propagatoin, it has a DB that can resonate

Radical Autooxidation reagents

O2, initiator

What is the product of radical autooxidation?

The O-OH group is added to the allylic position

What are the steps of a radical autooxidation

1. X radical pair with H bond radical, so radical left on C

2. O-O both with radicals goes in and replaces radical on C

3. Use H-R to take the radical off the O and pair it with the H, and break the H-R bond

What causes radical chain mechanisms?

Peroxides

HBr and peroxide added to alkene

Initiation (2 steps)

1. Break up peroxide into 2 with radicals

2. Add H-X, take the H and leave Br radical

Propagation

1. Add Br radical to molecule and break the DB to form a C radical (Br on the less sub side, C radical on more sub)

2. Use HBr to get rid of C radical and break Br radical again

Termination: C rad and Br rad join to get Br on both side of DB (NOT A PRODUCT)

C-X bonds are always

polar

What is polarizability

the ability of an atom to donate it’s e’s

Trend of halide size and polarizability

Polarizability increases and halide size increases

Branching increases or decreases BP

Branching decreases boiling point

Which has greater BP and why? Haloalkane v alkane

Haloalkane bc of dipoles and increased polarizability

Which has greater density? Haloalkane or alkane

Haloalkane

Bond length and strength: C-X vs. C-H

C-X is longer, so it is weaker (expcetion is C-F is stronger than C-h)

What does a larger halide mean in terms of bonds?

It means a longer bond so a weaker bond

What does a nucleophilic substitution do

Replaecs LG with a nucleophile

What does Nucleophilif substitution do to alkyl halides

convert it to other functional groups

What does B-elimination reactions do

Remove LG and Hydrogen and form a pi bond

B-elimination converts alkyl halides to

alkenes

Alkyl Halide Reactivity trend

I>Br>Cl»F

What is a protic solvent

has H bonding

What does a nucleophile do

donates e’s

What takes prioritity, a nucleophilic reaction or A/B?

A/B because it is faster

Nucleophilicity trend with Electronegativity

Nucleophilicity increases as EN decreases because the stability of the anion decreases which means it is more reactive

Are larger nucleophiles weaker or stronger and why

They are weaker because it is harder to attack the electrophile

SN2 stands for

substitution, nucleophilic, bimolecular

What happens to sterochem in SN2

Electrophile/product stereochem are switched

What happens in the concerted step of SN2?

The nucleophile attached the C and the LG leaves

Rate expression for Sn2

k [RX][nuc]

Alkyl Halide preference for SN2

1>2»»»3 (needs to easily reach C)

If there is a LG and OH, it is probably a

A/B rxn

What happens faster, inter or intra

intramolecular reactions

What does SN1 stand for

Substitution, nucleophilic, unimolecular

What is solvolysis

Solvent acts as a nucleotide

Does the SN1 nucleophile attack the electrophile?

No, it waits for the LG to leave (RDS)

SN1 rate expression

rate = k [RX]

Alkyl Halide preference for SN1

3 > 2 > 1 »» MeX

What increases stability in alkenes

Higher substitution

What is more stable, trans or cis

Trans, except in small cycloalkanes

If 2 molecules are similar, what do you prefer in terms of alkene stability

substitution > sterics

Elimination reactions are a type of what and what do they produce

Dehydrohalogenation and they produce pi bonds

What does E1 reaction stand for

Elimination, Unimolecular

In an E1 reaction, when a C+ is formed after the LG leaves, where do you pull off the proton?

The C+ is on a, so you pull it off on B (the -B charge can donate to the + a charge)

E1 rate expression

k[RX (LG]

E1 alkyl halide preference

3>2»»1

What does E2 stand for?

Elimination, Bimolecular