Chapter 7: Alkenes & Alkynes

What does this sheet show?

horizontal & vertical trends

Alkenes =

contain a carbon-carbon double bond

Alkynes =

contain a carbon-carbon triple bond

E/Z system is used for:

double bonds with two or more substituents

E =

the higher priority groups are on different sides

Z =

the higher priority groups are on the same side

Highest priority goes to the atom with:

the largest atomic number

What is used to measure the stability of alkenes?

Heat of hydrogenation

In general, the more substituted =

the more stable

Which alkenes have the highest & lowest heat of hydrogenation?

Lowest = RRRR — more stable

Highest = HHHH

Rings containing 5 or less carbons exist only in:

the cis form

Dehydrohalogenation =

remove hydrogen, remove halogen & put a double bone in its place

Which bases favor the zaitsev (more substituted alkene) product?

-OH

• NaOH & KOH

RO-

• NaOCH3

• NAOEt

*small strong bases*

Which bases favor the Hofmann product (least substituted)?

KotBu — large strong base

Elimination E2:

• takes place in one step

• Two components leave at the same time (bimolecular reaction)

• alkene that is formed follows Zaitsev’s rule

• rate = k[alkyl halide][base]

What does the reaction graph look like?

image

Antiperiplanar =

opposite sides, sample plane

For E2, the base must push out the leaving group from:

the opposite side & in the same plane → this is called anti-coplanar (or anti-periplanar)

For newman, how do you know eclipse vs staggered?

Eclipse = Dashes pair or wedges pair are on same side

Staggered = pair is on different sides of bond you are looking down on

How do you do an E2 reaction?

look at all connected carbons, pick the hydrogen that is in the same plane but opposite side — that tells placement of double bond

When will Newman rotation need to be used for E2?

if the leaving group and hydrogen are on the same side but not same plane

E2 is:

stereospecific

To practice chair to ring look on slide:

21

Common bases that result in E2 reactions are:

–OH and –OR

Examples of bases: NaOH, KOH, NaOEt, and NaOMe

Elimination: E1

• takes place in more than one step

• A carbocation intermediate forms (same as with the SN1 reaction)

• Tertiary alkyl halides work the best

• Rate = k[alkyl halide]

  • same for SN1 and E1

• Zaitsev only

What role does temperature play?

low temp = SN1

high temp = E1

H2O in E1:

• if acting as a Nu, it attacks the carbocation directly

• if acting as a base, it takes proton

For methyl halides:

Only SN2 possible

For 1° alkyl halides:

• SN2 mainly

• E2 if strong, sterically hindered base used (like potassium t-butoxide)

For 2° alkyl halides: SN2

with weakly basic nucleophiles

I–, Br–, CH3COO– = AcO–, –CN, or –N3

For 2° alkyl halides: E2

if a strong base is used

Zaitsev w/ NaOH, NaOMe, or NaOEt

Hofmann w/ KOtBu

For 2° alkyl halides: SN1 & E1

Low temp = SN1

High temp = E1

w/ solvents with H2O, MeOH, EtOH

For 3° alkyl halides: E2

• No SN2 possible

• E2 if a strong base is used (Zaitsev w/ NaOH, NaOMe, or NaOEt or Hofmann w/ KOtBu)

For 3° alkyl halides: SN1 or E1

Low temp = SN1

High temp = E1

w/ solvents with H2O, MeOH, EtOH

Dehydrohalogenation that proceeds by an ____ mechanism is the best for synthesizing alkenes?

E2

Dehydrohalogenation products?

can yield both Zaitsev’s and Hofmann products

Acid Catalyzed Dehydration of Alcohols:

• Alcohols undergo dehydration to give alkenes

• The most substituted alkene will be the major product

If it’s a tertiary or secondary alcohol it will proceed by:

E1

if it’s a primary alcohol, it will proceed by:

E2

Acid Catalyzed Dehydration of Alcohols temperature trend:

primary will need very high heat, secondary is less, and tertiary is even lower

Some 1° & 2° alcohols can undergo:

rearrangement

Carbocations can move:

only once

Hydride shift?

A H shifts to make the carbocation more stable

Methyl shift:

methyl group shifts to make carbocat more stable

Ring expansions:

ring opens up and expands to make carbocat more stable

Ring expansions that can happen:

3C to 4C

4C to 5C

5C to 6C is maybe

Ring expansion that won’t happen:

6C to 7C

Once the carbocation is stabilized by the shift, what happens?

final double bond forms and gets rid of carbocation

Sodium amide (NaNH2) is used to:

deprotonate a terminal alkyne — this is how you lengthen carbon chains

After a terminal alkyne has been deprotonated:

use either a methyl or or primary alkyl halide to do SN2

Synthesis of alkynes method 1:

• Start with a vicinal dibromide

  • Vicinal means adjacent or 3 bonds away

• Do two E2 eliminations

Synthesis of alkynes method 2:

• Prepare a geminal dihalide from a methyl ketone

• then do 2 E2 eliminations

Hydrogenation of Alkenes:

• addition of H2 across the double bond

• syn addition

Syn addition =

same side

Anti addition =

opposite sides

Catalytic hydrogenation goes by:

syn addition; both hydrogens add to the same side (or face)

Halogens will add to an alkene via:

anti addition; both halogens add to opposite sides (or faces)

To convert an alkyne into a cis-alkene:

use lindlar’s catalyst

To convert an alkyne into a trans-alkene:

use Li or Sodium Metal, NHs(l)

To convert an alkyne to an alkane:

two equivalents of H2 must be used with Pt, Pd or Ni

• this removes all pi bonds