OChem Unit 3 (Alkenes/Alkynes)

reactions of conjugated dienes

1) pi electrons attack the electrophile, forming resonance stabilized carbocation intermediate

2) the nucleophile attacks both carbocations, forming a 1-2 (direct) product and a 1-4 (indirect) product

major vs minor products

hydride/alkyl shifts

hydride shift - hydrogen shifts down a carbon to switch which carbon has a positive charge

alkyl shift - same as hydride shift but an alkyl group shifts

hydration of alkenes

additional of water to an alkene

new sigma bonds form to h and oh

requires an acid catalyst

acetylene

propylene

diels-alder reactions (cycloaddition)

a conjugated diene reacts with another molecule that has a pi bond to form a ring structure

bond breaking and making occur simultaneously

no carbocation intermediate

oxidation with permanganate in an acidic solution

breaks c=c bond and forms 2 c=o bonds

“snip and cap”

produces carboxylic acids and ketones

aldehydes are oxidized to carboxylic acids (h becomes oh)

alkene hydrogenation example

isopropyl

cumulated

(e) / (z)

distinguish between tri or tetra substituted configurational alkenes

(e)- higher priority groups on opposite sides

(z)- higher priority groups on same side

the higher priority group has the higher atomic number at the first point of difference

oxidation with permanganate in a basic solution

breaks c=c bond and adds oh groups to each carbon

also called dihydroxylation

syn addition (oh groups add to same side of ring)

produces glycol (compound with 2 adjacent oh groups)

hydrogenation of alkenes

additional of h2 to an alkene

2 new sigma bonds to hydrogens

hydrogens add to same side of a ring

requires a catalyst (ni, pt, pd)

difference between acid catalyzed hydration and hydroboration oxidation of alkenes

acid catalyzed hydration - forms markovnikov product (oh on more substituted carbon)

hydroboration oxidation - forms anti-markovnikov product (oh on less substituted carbon)

alkene halogenation

additional of a halogen to an alkene

2 new sigma bonds to halogen

halogens add to opposite sides of a ring

allyl

alkene hydroboration-oxidation reactions

isolated/non-conjugated

ozonolysis

breaks c=c bond and forms 2 c=o bonds

forms aldehydes and ketones

1st step) ozone (o3)

2nd step) zinc in acid (zn;h+)

methylene

sec-butyl

alkene halogenation example

acid reaction with an alkene

new sigma bond with hydrogen

new sigma bond with anion

cis/trans

distinguish between disubstituted configurational alkene isomers

cis - same side

trans - opposite sides

isobutyl

electrophilic addition mechanisms

1) pi electrons attack electrophile and form carbocation intermediate

2) nucleophile attacks carbocation

3 [only occurs in acid catalyzed hydration]) product loses a proton to regenerate catalyst (deprotonization)

ethylene

vinyl

conjugated

alkene hydration example

regioselective vs regiospecific

regioselective - both major and minor products form

regiospecific - only one product forms

1 vs 2 equivalents

1 equivalent - is the amount of reagent needed to react with 1 mole of reactant, 1 pi bond is broken

2 equivalents - twice the amount of reagent as 1 equivalent, 2 pi bonds are broken

halogenation of alkynes

1 equivalent - 2 halogens are added on opposite sides, 1 pi bond is broken

2 equivalents - 4 halogens added, 2 pi bonds broken

Ni/Pt vs Lindlar’s catalyst

both catalysts for hydrogenation of alkynes

Ni/Pt - produces alkane (complete hydrogenation)

Lindlar’s constant - produces alkene (partial hydrogenation)

hydrohalogenation of alkynes

electrophilic addition of hydrogen halides (hx)

markovnikov addition (x adds to more substituted carbon)

hydrohalogenation of unsymmetrical alkynes leads to isomers

hydration of alkynes

hydroboration-oxidation of alkynes

difference between hydration and hydroboration oxidation of alkynes

hydration forms markovnikov product (double bond to oxygen on the more substituted carbon)

hydroboration oxidation forms anti-markovnikov product (double bond to oxygen on the less substituted carbon)

carboxylic acid

glycol

a type of alcohol where 2 hydroxyl groups are on adjacent carbons

aldehyde

ketone