Ch. 10 Conjugation In Alkadienes And Allylic Systems

Front 1

Carbocation stability from most stable to least

Back 1

tertiary > secondary > primary> methyl

Front 2

allyl group

Back 2

H2C = CHCH2-

Front 3

Allylic carbon

Back 3

carbon at the end of the Allylic system that is sp3 hybridized; anything attached to this carbon are Allylic substituents

Front 4

vinylic substituents

Back 4

substituents attached to c=c

Front 5

Carbocations which have the positive charge on an Allylic carbon are resonance stabilized. Positive charge shared between carbons 1 and 3 not 2.

Back 5

know

Front 6

Allylic carbocations are more stable than their simple alkyl cation siblings

Back 6

know

Front 7

Remember the more stable the intermediate carbocation the more SN1 character we see in the product mixture

Back 7

know

Front 8

Nucleophilic attack on Allylic carbocation

Back 8

Since there are two carbons which share positive charge, (due to resonance) there are two potential sites for Nucleophile to attack. The Nucleophile will react with both carbons.

Front 9

Regioselectiveness for Nucleophilic attack on Allylic carbocations

Back 9

The preference will be for the most stable Allylic carbocation of the two carbons(from the resonance Allylic carbocation). The most preferred is for a tertiary carbocation which will be your major product. The least stable of the two will be your minor product

Front 10

SN1 Hydrolysis of Allylic halide

Back 10

Turns Allylic halide into allylic alcohol.
Allylic halide plus water will give two products :
1. Major product will be OH reacting with stable carbon with positive charge(tertiary or best)
2. Minor product will be OH on primary pos carbon

Front 11

Mechanism of SN1 Hydrolysis of Allylic Halide

Back 11

1. Leaving group leaves to form Allylic carbocation ( Resonance gives hybrid of 2 since positive charge is shared between 2 carbons due to delocalization of electrons)
2. Carbocation reacts with H20 and bonds to either tertiary of primary carbon of Allylic carbocation.
3. alkyloxonium ion formed converted to alcohols by proton transfer. Will get your major and minor products

Front 12

Stability of Allylic Free Radicals

Back 12

reactions that produce Allylic free radicals proceed more readily than those that give simple alkyl radicals which proves their higher stability

Front 13

Allylic Chlorination or Bromination

Back 13

two ways reaction can proceed:
1. Room temp cl and br will react with alkenes by anti addition product(
2. at elevated temps(500 deg) substitution reacts compete so reaction will undergo substitution product

Front 14

Hydrolysis of Allylic halide can also proceed by sn2 where primary product will major product

Back 14

typically will occur with primary and unhindered secondary Allylic halides react with good nucleophiles

NaOCh2CH3 over ethanol

Front 15

Allylic chlorination or bromination
allyl + cl2 or br2--------> room temps gives

Back 15

anti addition product cl or br will be attached to each carbon around double bond one up one down

Front 16

allyl + br2 or cl2 at high temps (500 deg)

Back 16

substitution product (free radical mechanism) cl or br will be where proton(h) was taken from Allylic carbon to form Allylic radical(if equivalent only one. product but if not can be two products

Front 17

Allylic bromination
allyl plus NBS with heat and CCl4

Back 17

br reacts at Allylic carbon position and substitutes there and no where else! only one product!

Front 18

Free Radical Halogenation
Allylic chlorination mechanism
(cl2 at 500deg)

Back 18

1. initiation step: cl2 seperate and give one electron to each cl giving 2 cl radicals
2. prop steps: cl radical takes a h atom from Allylic carbon which forms Allylic radical carbocation
3. another cl 2 comes in seperates (like initiation step) and forms with cation to give an allylic chloride and a free radical

Front 19

Limitations for Allylic free radical halogenations

Back 19

these conditions must be met otherwise you'll get a mixture of products due to resonance of Allylic radical carbocation
1. all allylic hydrogens must be equivalent( do sub test)with naming
2. both resonance forms of Allylic radical carbocation must be equivalent( primary to primary, secondary to secondary etc)

Front 20

isolated diene

Back 20

double bonds separated by Atleast one sp3 carbon

Front 21

conjugated diene

Back 21

double bonded carbons connected by a single bond ( if go along chain all will be sp2 in a row )

Front 22

cumulated diene or Allene

Back 22

double bonds share a common carbon which will be sp

Front 23

stability of dines from most stable to least

Back 23

conjugated>isolated> cumulated

Front 24

s-cis vs s-trans conformation in dienes

Back 24

can be either in A diene but trans is most stable. cis is least stable due to steric interference between hydrogen s from carbons 1 and 4

Front 25

Making dienes: to make diene from unsaturated alcohol(double bonded) use elimination reactions

Back 25

alkene with oh group + KHSO4 and heat
dehydration s are regioselectiveness towards most stable double bond, ( conjugated preferred over isolated)
beta elimination so assign alpha beta carbons and place double bond to make out stable will be primary, others will be minor

Front 26

making diene: dehydrohalogenation reaction

alkene with halogen plus KOH with heat gives?

Back 26

same as dehydration of alcohol but instead of alkene with oh group you will have a halogen: br , cl, f, I
Beta elimination regioselectiveness towards most stable double bond will be major product.

Front 27

making isolated dienes: possible when structure of alkyl halide or alcohol prevent formation of conjugated double bond such as awhen a bridged group is present

Back 27

bridged group alkyl halide or alcohol plus KOC(CH3) with DMSO at 70 deg gives isolated diene

Front 28

What reactions can you make dienes?

Back 28

beta elimination reactions of dehydration of alcohols and alkyl halides

Front 29

Addition of hydrogen halides to conjugated dienes undergo electrophilic addition reactions

Back 29

the regioselectivity of electrophilic addition to conjugated dienes is governed by the stability of the resulting carbocation. Protonation of a conjugated diene always occurs at the end of the diene unit because an allylic carbocation results.
will get one product if resonance is equivalent but get two if they're not

Front 30

hydrogen halide to conjugated diene if resonance equivalent( meaning positive charge on either carbon gives same product when attacked)

Back 30

you'll get one product 12 addition

Front 31

when Allylic carbocation intermediates are not equivalent what products will u get

Back 31

1,2 addition and 1,4 addition ( depends on temp which one will be major or minor)

Front 32

conj diene plus hbr/hcl -80 degrees

Back 32

gives kinetic product: 1,2 addition will be major product and 1,4 addition will be minor

Front 33

conj diene plus HBr/ HCl plus room temp gives

Back 33

thermodynamic control so 1,4 addition will be major product and 1,2 will be minor

Front 34

halogen addition to dienes
diene plus cl2/ br2 plus non competing solvent like chcl3 or cl4 at room temp

Back 34

you'll get both added halogen to 1,2 and 1,4 addition products but 1,4 is preferred and e double bonds preferred

Front 35

diene stability

Back 35

although s trans is more stable than s cis, diene must be in s cis for diels Adler reaction
diene in a ring most stable (locked in) > diene with alkyl grps on carbons 2 and 3> alkyl grps on carbons 1 and 4 pointing out > diene with alkyl grps on carbons 1 and 4 pointing in( least stable due to sterics from h)

Front 36

electron donating groups on diene increase reactivity

Back 36

look for alkyl grps with o,n,c
most stable to least stable:
diene with electron donating grp more stable than plain diene more stable than diene with electron donating grp pointing out

Front 37

Diels aler reaction is completely stereo specific at diene and dienophile. If start with cis dienophile end with cis DA product around where double bond was (A and B) Same for Trans .

Back 37

Grps off c1 and c4 of diene maintain stereochemistry as well. If both products pointing in or out = cis. If 1 in and 1 out= trans

Front 38

Stereochemistry of DA rxn

Back 38

when there is a choice between endo or exo addition the endo product is preferred
Endo product also preferred under kinetic conditions
Exo product preferred under thermodynamic

Front 39

Regiochemistry of DA reaction

assign partial charges than line up! look at product where it reacted between substituents that's how u bet the numbers

Back 39

when you have an unsymmetrical diene and an unsymmetrical dienophile, there is a possibility o f two products

1,4 is preferred over 1,3
1,2 is preferred over 1,3

never choose 1,3: partial charges don't line up!