Kaplan MCAT OChem Ch. 5: Alkenes, Alkynes, and Elimination Reactions

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unsat. fats

fatty carbon chains w/ one or more DBs

double bonds

considered func. groups

alkenes

more reactive than alkanes

simplest alkene

ethene (ethylene)

alkynes

at least one triple bond

alkynes

unstable and very reactive

alkenes and alkynes

contain pi bonds => many similar properties

pi bonds

commonly formed through elimination rxns

alkenes

sometimes called olefins

alkenes

described cis, trans, E, and Z

ethylene

https://www.softchalkcloud.com/lesson/files/E4hyMb8JpR79i3/Ethylene-2D.png

isobutylene

http://www.wavesignal.com/o_chem/images/Alkene10.gif

propylene

http://upload.wikimedia.org/wikipedia/commons/d/d1/Propylene.PNG

alkenes

physical properties are similar to those of alkanes

trans-alkenes

have higher MP that cis since more symm => better packing in solid state

trans-alkenes

have lower BP than cis alkenes since less polar

polarity

asymm dist'n of electrons in a molecule => molecule has one partially neg. region and one partially pos. region

alkenes

unequal electron dist'n => creates dipole moments pointing from electropositive alkyl groups toward electronegative alkene (sp3 donates to sp2, 3 has less s-character than sp2, s can be found at pos. nucleus => more stable)

trans-2-butene

two dipole moments are oriented in opposite directions => cancel each other => no net dipole moment and nonpolar

cis-2-butene

addition of two smaller dipoles => has net dipole moment

alkene

polarity => add. intermolecular forces => raise BP

two mechs of elimination (unimolecular and bimolecular elimination )

e1 and e2

elimination rxns

way to synthesize alkenes, of either alcohols or alkyl halides

elimination rxns

carbon backbone kicks off (or eliminates) a H and halide (dehydrohalogenation) or a molecule of water (dehydration) => DB

dehydrohalogenation

C backbone kicks off H and halide

dehydration

C backbone kicks off water

E1 (unimolecular elimination)

two step process

rate of reactions depends on conc. of only one species, the substrate

E1 (unimolecular elimination) steps

1. LG leaves and form of carbocation

2. proton on adjacent carbon (beta- carbon) is moved by WB => DB formed

E2 (bimolecular elimination)

if strong base present, this is more likely

E1 (unimolecular elimination) factors

polar protic solvents

ability to form stable carbocation

highly branched carbon chains

good LGs

absence of a good Nu

E1 (unimolecular elimination)

higher temps favor this pathway

E2 (bimolecular elimination)

...

E1 (unimolecular elimination)

typically favor that more stable alkenes as major product (more highly sub)

E2 (bimolecular elimination)

one step

rate depends on two species, the sub and the base (Nu)

E2 (bimolecular elimination)

SB (ex: ethoxide ion) removes proton => halide ion anti to proton leaves => DB

E2 (bimolecular elimination)

has two possible prod.

DB can form on either side of departing halide, but more sub. DB is large % of products; if can form either geo isomers, trans predominates since more stable

E2 vs. Sn2 (e2)

steric hindrance is imortant

highly sub carbon chains (form stable alkenes), undergo this and other rarely

E2 vs. Sn2 (e2)

bulk of base (Nu) has hard time getting to backside of alpha carbon (w/ LG attached)

much easier to pluck off H from neighboring chain

E2 vs. Sn2 (e2)

strong base favor this over other

pull off a beta-H before can reach alpha-carbon => this rxn

E2 vs. Sn2 (sn2)

weak Lewis base (strong Nu) favored

E1 vs. Sn1 (e1)

done by controlling conditions, like factors like polarity of solvent, or most important, the temp.

e1

least likely mech of the four

Reactions of alkenes

reduction

electrophilic addition

addition of HX

addition of x(2)

add of h20

free radical add.

hydroboration

oxidation

types of oxidation in alkenes

potassium permanganate

ozonolysis

percarboxylic acids

polymerization

oxidation

loses H => form DB

often gains O

reduction

getting more H

type of reduction rxn

catalytic hydrogenation

catalytic hydrogenation

reducing alkene by adding molecular H to DB w/ aid of metal catalyst

typical catalysts of catalytic hydrogenation

Pt, Pd, Ni, sometimes rhodium, iridium, or ruthenium

catalytic hydrogenation

takes place on surface of metal

one face of pi bond becomes coordinated to the metal surface where molecular H is bond => rxn takes place where two touch (syn addition)

catalytic hydrogenation

this type of addition known as syn addition

stereospecific rxns

reaction where only one stereoisomer is formed

type of electrophilic addition rxns (alkenes)

addition of HX

addition of x(2)

add of h20

electrophilic addition rxns (alkenes)

pi bond broken w/o breaking the sigma bond (add rxns)

electrophilic addition rxns (alkenes)

since electrons of pi bond are particularly reactive => easily attacked by molecules seeking to accept electron pair (Lewis acids, electrophiles)

electrophiles

lovers of electrons

Lewis Acids

Accept Electrons

Lewis Bases

donate electrons

addition of HX

DB acts like Lewis base and reacts w/ partially positive hydrogen of HX

addition of HX

first step yields carbocation intermediate

addition of HX

in cases where alkene is asymm => initial protonation produces most stable carbocation

alkyl sub. stabilize carbocations

forms slow step

addition of HX

Mark's rule

Mark's rule

produce most stable carbocation

addition of HX

2nd step, halide ion combines w/ carbocation => alkyl halide

addition of X2

addition of this to DB is a rapid process

addition of X2

used a diagnostic tool to test presence of DB

addition of X2

1st step: DB as N => attacks 1/2 of X2 => X- as LG and cyclic halonium ion that dissipates pos charge w/ trinuclear intermediate

addition of X2

2nd step: X- attacks ion on on opposite face => dihalo compound

addition of X2

if rxn in Nu solvent, cyclic halonium ion attacked by this first before halogen ion => halo alcohol

Addition of H2O

water added to DB under acidic conditions (sulfuric acid)

Addition of H2O

DB protonated by Mark's rule => reacts w/ water => protoanted alcohol => lose proton => alcohol

Addition of H2O

performed in low temp, since at high, reversed rxn favored (dehydration)

Addition of H2O

can also be achieved under mild conditions w/ oxymercuration-reduction

free radical additions

add HX to DB using free radical intermediates

free radical additions

disobey Mark's rule because X radical add first to DB => most stable free radical, making halogen end up on least sub. carbon

free radical additions

useful for HBR, not for HI and HCl since energetically unfavorable

always determines favored products

most stable intermediate and least energetic transition state

hydroboration

first step: B2H6 (diborane) adds to DB => B atom (LA) to less sterically hindered, at same time H transferred to adjacent carbon

free radical additions

second step: oxidation-hydrolysis w/ peroxide (H2O2) and aq. base (-OH) => transfers water to bond w/ born => alcohol

oxidation

if reagent has bunch of oxygen => changes that it's an oxidizing agent

potassium permanganate (oxidation) KMnO4

alkenes ca n be oxidized w/ this, depending on rxn conditions => end w/ diff products

potassium permanganate (oxidation) KMnO4

if condition mild, using cold, dilute, basic KMnO4 => 1,2 diols (vicinal diols)

vicinal diols or glycols

1,2 diols

syn addition

potassium permanganate (oxidation) KMnO4

using hot, basic, sol of this, followed by an acid wash

1. nonterminal cleaved => to form two molar equivalents of carb acid
2. terminal cleaved => carb acid and CO2
4. if nonterminal is disub => ketone formed

ozonolysis

more selective than hote, acidic, KMnO4

ozonolysis

cleaves DB, but only oxidizes the carbon to aldehyde (or ketone if starting molecule is disub)

ozonolysis

under reducing conditions (Zn/H+ or (CH3)2S

ozonolysis

under oxidizing conditions (H peroxide) yields same products as hot, acidic KMnO4

ozonolysis

can obtain alcohols if reduce aldehyde or ketone products w/ mild reducing agent, such as NaBH4 or LiAH4

percarb acids

alkenes oxidized by this, which are strong oxidizign agents

percarb acids

peroxyacetic acid (CH3CO3H) and MCPBA commonly used

percarb acids

products are epoxides (oxiranes)

percarb acids

syn additon

epoxides

products of percarb acids

polymerization

creation of long, high MW chains (polymers) composed of monomers

polymerization

usually occur through a radical mech, although anionic and even cationic are observed too

monomers

repeating units that polymers are composed of

polymerization

some require high temp and pressure

heat present

consider possible radical mech

alkynes

one or more C-C triple bonds

alkynes

180 degrees straight lines from sp hybridization

common name from ethyne

acetylene

physical properties of alkynes

to make triple bonds, go through two rounds of elimination of geminal (twins) or vicinal dihalides (requires high temp and SB though)

physical properties of alkynes

more useful method is adding already existing triple bon into a new carbon skeleton

terminal TB coverted into Nu by reomving acidic H w/ SB (NaNH2 or n-BuLi) => acetylide ion => ion performs Nu displacement on primary alkyl halides at room temp.

terminal alkynes

are fairly acidic

rxns of alkynes

reductions

addtions

hydroboration

oxidation

type of addition rxns of alkynes

electrophilic

free radical

reduction (alkynes)

alkynes can be hydrogenated (reduced) w/ catalyst to give alkanes

if just want alkenes => stop this after one equivalent of H (partial hydrogenation)

reduction (alkynes)

use Lindlar's catalyst, w/ palladium on BaSO4 w/ quinoline, a heterocyclic aromatic poison

reduction (alkynes)

w/ Lindlar's catalyst, since rxn occurs on metal surface, product is cis-isomer

reduction (alkynes)

second method (just to alkene), use Na in liquid ammonia at temp. below -33 C (BP of ammonia) => trans isomer via free radical mech

addition, electrophilic (alkynes)

follows Mark's rule

addition, electrophilic (alkynes)

can be stopped at intermediate (alkene)

addition, electrophilic (alkynes)

can go all the way to alkane, just need two equivalents

free radical, electrophilic (alkynes)

anti-Mark

free radical, electrophilic (alkynes)

reaction product is usually trans-isomers, since intermediate vinyl radical can isomerize to its more stable form

hydroboration

addition of boron on TB => syn

hydroboration

boron bound to 3 diff sub

hydroboration

boron addition followed by acetic acid wash, boron atom removed => each sub w/ have proton from acetic acid => cis-alkene

hydroboration

w/ terminal alkynes, disub borane used to prevent further boration of vinylic intermediate to an alkane => vinylic borane can be oxidatively cleaved w/ H2O2, creating an intermediate vinyl alcohol (an enol) => tautomerizes to more stable carbonyl compound (via keto-enol taut)

oxidation (alkynes)

can be oxidatively cleaved w/ either hot, basic, KMnO4 (followed by acidification) or w/ ozone

oxidation (alkenes)

w/ ozone reducing cond (Zn/CH3COOH) => aldehyde or ketones

oxidizing condition (H2O2) => carb acids

oxidation (alkynes)

w/ ozone - carb acid or CO2

TB adds two oxygne to each carbon

terminam alkyne - CO2

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