CHAPTER SOLUTIONS >Chapter 10: Radical Reactions:_University of North Texas CHEM 3210

Chapter 10: Radical Reactions

fit PEEK : Monday

November 9 , 2020

10.1 Radicals - Introduction •

free radicals

K¥1 # Elad 110011 &

Time 00:44

form when bonds break hemolytically

•

Radicals

HOMO lytic bond cleavage

-

=

species with Unpaired electrons

arise from HOMO lytic bond cleavage

lytic bond Clevage = bond between 2 atoms breaks { each atom gets 1 Of the e that were in the bond

↳ HOMO

-

.

•

Hetero

lytic bond clevages When I atom gets both e G the other -

gets none forming ,

•

Note the

single

-

ions

.

barbed or fishhook arrow used to show the electron movement

rn y

x

full head -

arrow

1'

half headed -

Recall the orbital

arrow 1

Time 04:31

10.1 Radical Structure and Geometry •

-

hybridization in carbocation s 4 Carb anions Radical

-

Is

somewhere in the Middle

-

-

3e

groups

-

-

-

10.1 Radical Structure and Geometry •

Time 05:53

Radicals appear to be trigonal planar Gp2 hybridized ) Or

shallow

trigonal planar ( sp 's hybridized) Assume :

carbon radical = Spl ↳

's

sp

unpaired e

-

will be in pure p orbital

10.1 Free Radical Stability •

•

Radicals

=

neutral

=

Time 09:02 no formal

charge

=

still e

-

deficient tho

=

incomplete octet

Radicals follow the same stabllty trend as Carbo cations b/c they are both e f. C :

•

V. f

-

lines

↳ methyl : 4 7- e •

-

=

d. efficient species

-

dots

-

3

-

-

I

=D ; still e deff Kent bk Only has -

Incomplete octet

more substituted radical Is , the more stable HIS

groups stabilize e defficient carbons via hyperconjugation ↳ can also be stabilized by resonance

↳R

-

10.1 Free Radical Resonance Time 11:29 •

•

Radicals , like carbocation s

=

arrows used to

Fishhook

stabilized by resonance delocalization

present possible resonance forms -

need 3 half arrows : -

-

21-0 Show a bond breaking homolytl Lally 3rd to show how the unpaired e

-

is now pairing up with one

of the other e

-

H H l

l

H l →

,

H

It

"

I

.

It

.

H

.

H

10.1 Radicals Time 14:54 Need to be able to this ; make sure you dont create extra unpaired e " Practice: Draw resonance structures of the radical species below.

::c:: :c: intimate:#are:.io: :

•

•÷ w

-

g

e •

U

e

s

-

-

.

A

•

•

The

÷

)

.

minimizes repulsions

'

Radical somewhat -

unstable but resonance spreads out radical

through structure , making it more stable '

:

.

.

hybrid

Time 22:24

more resonance delocalized a radical is , the more stable It is

Benzylic

radicals

=

MORE stable than allylic radicals bk the radical is delocalized over more carbon atoms •

x

Ky x

vinyl IC

Phen HIC

•

* stability

benzylic > allylic > 307 20

7

10

>

methyl

>

" ""

.

.

//÷÷ /

•

resonance

10.1 Free Radical Resonance

.

hybrid

Resonance

( o.ro

.

-

Vinyl k > phehllk

.

Time 26:08

10.1 Resonance Stabilization Resonance stabilized radicals

•

=

more

stable than

30 radicals

XXX

-

BD C. ↳

=

Bond dissociation energy

→

X

.

.

X

energy required to break bond HOMO lytically , forming 2 radicals

↳

more stable the to form It

10.1 Radical Stability Vinyl k radicals

•

=

resisting radical is , the less energy it takes

.

Time 29:51

especially

Unstable (as with Vinyl cations)

Asking same thing kinda

Identify most

*

On Exam he may ask you to

(

stable 4 least stable , Use this :

benzylic > allylic 730 7207 10 > Methyl > vinyl k > phenllk * He may also ask you to rank C H bonds with regards to their strength -

An alkene radical ( VinylIC)

•

=

less stable than any alkane radical

Practice Time 31:29 Rank the indicated carbon-hydrogen bonds from the STRONGEST BOND to the WEAKEST BOND. -

I

(

r

N

u

c

U

Radical stability

•

Bond strength

D

bond

•

E

•

=

(strongest bond - MOST energy to form

LEAST energy to form

T

10.2 Patterns in Radical Mechanisms

"

like

°

But radical mechanisms

•

Time 37:44

distinctly different Ex : radicals don't undergo rearrangement =

Radical mechanisms follow

Most stable radical)

weakest bk most stable radical

10h11 Mechanisms , radical mechanisms follow patterns

•

=

D > A > B >E > C

strongest

-

.

( ( benzylic) > C- (allylic) > B 129 , A 119 , D ( vinyl lb)

•

,

,

2

B

A

homolytlcally d. see what type of radicals you end up creating

then rank strengths

o Z

Break all bonds

BDE Problem

6

.

key arrow pushing patterns

=

least stable radical ; weakest

10.2 Patterns in Radical Mechanisms 7. HOMO lytic

Time 39:11

clevage : initiated by light or heat

Breaking bond

-

o 2

•

4

5

.

.

.

light

combining 2 things

Halogen abstraction

of addition

Elimination : The reverse

The radical on the

6. coupling : the reverse

a

carbon is pushed toward the B carbon to eliminate a radical atom 1 group

of hemolytic cleavage

10.2 Patterns in Radical Mechanisms Group the relationships)

The -

-

-

steps

in

Time 45:01

among these 6 patters

10.2 Patterns in Radical Mechanisms •

heat , hu =

endothermic

Hydrogen abstraction : not the same as a proton transfer

°

•

=

'

Addition to pl bond

.

3

-

a radical mechanism are

4 you 'll see there only 3 processes forward a backward

Time 46:43

,

Radical rxns

=

radical chain rxns

classified as either initiation termination or propagation

An initiation step = radical species are

,

formed from a non radical species -

,

→ starts Imitates

chain , creating radicals

Propagation occurs when radical reacts with a non radical species to form a new radical Cha non Termination

-

radical)

occurs when 2 radical species react to form a non radical species

need heat flight

happens after in H1 at 10h

end Of chain

10.3 Chlorination of Methane °

Time 50:16

=

halogen at ton

Chlorination Of methane follows a radical mechanism -

Radical mechanism consists

of 3 distinct

7. INITIATION : chlorine radicals

2.

alkane

→

alkyl halide

blueprint for similar type problems

stages :

are created

PROPAGATION : These steps = self sustaining The 1st -

.

produces one

3.

=

of an alkane =

propagation step consumes a Cl radical , 4 the 2nd step

.

TERMINATION : Occurs when radicals

collided OV pie

= .

←

major product

Stop

being radicals

we combine all the radicals that we formed In initiation 4

propagation in

←

10.3 Chlorination of Methane •

°

The sum of the propagation

Propagation

=

a

all possible Iterations

(combinations)

Side products

Time 01:00:03

steps gives the net reaction :

chain reaction : the product Isl for a later step serve as reactant Is)

for an earlier step in the mechanism

.

di

Time 01:01:54

10.3 Chlorination of Methane °

•

Poly chlorination

=

difficult to prevent

methyl chloride

=

more reactive towards radical halogen at ton than methane

°

Need excess methane , relative to Cle , for mono chlorination to be the major

product

.

Time 01:02:45 10.3 Chlorination of Methane Practice: Draw the complete mechanism for chlorination of ethane. Include all three stages (initiation, propagation, termination).

/

Initiation : Cl Cl bond breaks -

ii. :

Ii :

Iii

→

I

propagation :

n

•

it

•

i

:

l

: → "Iii

u

Termination

→

.

÷

:

.

→

ok .

,

.ci :

→:

-

Ii :

→

→

a

I

Iii

a

no

10.3 Radical Initiators Time 01:08:10 °

Radical initiator : posses es

a

cleaves HOMO lytically with heat or light

weak bond that

almost easy when we have single covalent bond between 2 Identical atoms -

°

The

Cl

-

Cl

-

,

Br

-

Bri RO OR

acyl peroxide = Most reactive his effective at 80°C ,

10.4 Halogenation Thermodynamics Time 01:10:19 °

•

•

if we want to determine whether a

in a

halogen at Ion rxn , entropy

# of reactants

=

=

process

=

product favored , we must determine the sign

ft l ) for OG -

negligible

# Of products

got

10.4 Halogenation Thermodynamics Time 01:11:29

←

too violent

←

non spontaneous

-

-

10.4 Halogenation Thermodynamics Time 01:12:47 •

•

°

Flo urination

=

so exothermic , Its not

10 donation = endothermic

Only halogen at con w Hh

=

practical 400 Violent)

not therm Odiham Cally favored

-

doesn't occur

Ck 4 Bra = useful

10.4 Halogenation Thermodynamics Time 01:13:18 •

Both chlorination h brominate on

=

exothermic

-

favored

chlorination → nonselective n

this

tru

brominate on → selective Bre

int

°

But , OH for the 1st step of brominate on

=

endothermic

←

( goes to more sub Side) .

Br

al

hence, selectiveness of Brom nation

10.4 Halogenation Thermodynamics Time 01:17:54 °

Brom nation

=

Much slower than chlorination = more selective

Propagator

propagation

\ 10 20

30

M

chlorine : any

alkyl Intermediate

=

down

hill G can form

bioiminenme:S:* :*: : ".in;

mediate

10.5 Halogenation Regioselectivity Time 01:19:24 •

•

with substrates more

complex than ethane , Mutt pie MONO halogen at ton products = possible

chlorine = indiscriminate

'

.

significant amount of each product is formed

nonselective

10.5 Halogenation Regioselectivity Time 01:20:59 °

But since 20 radicals

=

more stable than 10 , the 20 halide = most abundant product

know both are major Gg

Idf

-

-

10.5 Halogenation Regioselectivity Time 01:21:29 •

Bromnation

=

much slower , 450 Its much more

Maior

"

The

fact that

regio selective for the more reactive 20 carbon :

X

b MMI nation = more regio selective than chlorination can be explained by the Hammond Postulate

10.5 Halogenation Regioselectivity Time 01:22:08 Hammond postulate tells us that the transition state will most resemble the side that is closer to in the diagram ↳ for chlorination Ts State closest to reactants "

↳ for b nomination Ts State ,

closest to products so the

stability of the alkyl radical matters in brominateon but not In chlorination

.

Et

10.5 Halogenation Regioselectivity Time 01:23:23 •

stability

for Brom nation , the

Only form most stable alkyl radical

of the

transition state largely depends on the

stability of the resulting carbon radical

for chlorination , the

°

any alkyl radical can form

stability of the

transition state doesn't depend on the

stability of the resulting carbon radical so much

.

10.5 Halogenation Regioselectivity Time 01:24:16 •

In short the difference in TS ,

°

energies for the formation of a 10 us 20 us 30 radical

Thus , brominate on = more selective for the lower

=

much bigger for b MMM Atl On

energy pathway

0

10.5 Halogenation Regioselectivity Time 01:25:14 •

Brom nation

at the

30 position happens 1600 x more often than

at the

primary position

On Exam , he will give same alkane 4 ask you to draw all major Mono chlorination 0

-

O

H

products 9 all major

mono brominate on

products

-83

So

O

X 10.6 Halogenation Stereochemistry •

•

The halogenat ton of butane or more

2

-

chloro butane will

Time 01:26:24 complex alkanes forms

a new

chirality center

form a racemic mixture

" radical

-

-

trigonal planner so It can be attacked from either side

10.6 Halogenation Stereochemistry •

whether the free radical carbon

sp

-

Time 01:26:47 '

or a rapidly

Mterconvenlng Sp3, the halogen abstraction will occur on either side

of the plane with equal probability

10.6 Halogenation Stereochemistry °

There

Is actually a total Of

Time 01:27:03

3 monosubstltvted products

Time 01:27:05 Typical exam question Practice How many monochlorination products are possible?

-

formed in the halogen axon of butane

Nonselective

Cl

I

Cl

10

20

(

a

Ba ⑤

a

(

30

10

Cl

-

-

-

-

⑨

@ Cl 10 Not chiral -

@

a

Cl

-

chiral

E, ⑥

Ge

Cl

=

•

30 not chiral -

2010

-

chiral

SO , a major

products

How many monobromination products are possible?

it

A

µBr ✓ Br ~

only major ( more sub product) -

E

c,

I 20

③

@

put chirality If there

is

chirality ble racemic mixture

kit PEEK : Monday

•

Eigil 110011 $

November 16, 2020

10.7 Allylic Halogenation •

K¥1

Recall the weakest

Alkenes selectively

C

-

Time 00:32

H bond

110 West BDE) will undergo radical H

-

abstraction

faster than the others

.

undergo It abstraction at the allylic carbon Allylic halogen at ion placing halogen in an allylic position In a carbon -

=

connected to a double bond (Not

10.7 Allylic Halogenation •

•

H

abstraction

-

so

,

of an

placed directly on alkene)

.

Time 02:16 allylic H results In a resonance stabilized allylic radical :

mom , nation of an alkene is selective for the

A " Y ' " carbon

competing rxn :

Bragg

Br .

Br z •

But , addition Ot Br , to the alkene is a

10.7 Allylic Halogenation with NBS •

To

avoid the

competing rxn

.

.

.

Time 04:17

competing alkene addition , NBS Is used

as a source

rn °

Bro

Br

b

Br,

y

of Br radicals (instead Of Using Br z)

this can happen but

low concentration so no

competing side rxn 4 Only allylic halogen at ton occurs If we just used Bre .

instead Of NBS ,then both allylic

halogen at Ion da addition to alkene (competing rxn) occurs °

.

Using NBS keeps the concentration of HBR { Bre low , so competing addition reactions to the alkene are MINIMIZED

10.7 Allylic Halogenation with NBS °

•

Time 06:55

product

produced reacts with NBS to form Bre

NEED TO KNOW : the 2 propagation steps even

tho there Is Initiation , propagation , h termination

Propagation produces the allylic radical GH Br

The HBR

* ONLY

.

.

.

4 this Bre then continues propagation {the formation of the

NBS

Ex : symmetrical

Br

,

hv

alkene

Propagation : D. BIG '

n

2)

on

'

Br

-

HBR

µ

>

Br

s

,

U

hydrogen abstraction forming alkyl radical ,

.

halogen abstraction , theres a little bit of Br, be 2 Br radicals find eachother Goupil This -

Bro

Br

.

allows the 2nd

10.7 Allylic Halogenation with NBS •

•

propagation step to occur

.

Time 11:29

with asymmetric alkenes , a mixture of isomeric products is often Obtained

The radical

Is

delocalized on 2 Carbon atoms 450 halogenatlon occurs at either side ,

A- ssymetricaiex , Propagation

BI

a' •

Hut

:p of

f

:

•

s

s

resonance

s

Br

th

f

Ib

Br

Both are major products

10.7 Allylic Halogenation with NBS Time 16:15 Hhlnk Its best to just draw resonance to make sure Practice: Predict all possible organic product(s) for each reaction below. l product * symmetrical ( even tho looks asymmetrical) In -

.p÷ :

g.

( xn

I

g

as

-

→

@

Br

×

Br

Br e

Hugger

(

:

Br .

W

-

/

Br

I

.

q →

Br ⇐ =

10.10 Radical Addition of HBr •

we learned

In CH 9 that H

when peroxides are

Time 22:14 -

present

X will add across a c :C double bond with anti

-

Markovnikov

regioselectivity

.

* NEED TO KNOW ALL steps : INITIATION ,

propagation 4 termination ,

•

peroxides initiate

The

a

radical mechanism of addition

( Different mechanism , different

10.10 Anti Markovnikov Addition •

.

selectivity)

Time 28:01

The reaction IS Initiated by thermal hemolysis of the peroxide , which Initiates the formation of a Br radical from H Br

.

Need 2 initiation steps

AIKOXY radicals

10.10 Anti Markovnikov Addition •

Time 31:39

The Br radical then , in theory , could add to either side of the alkene :

In

H Br t RO OR adds Br to less substituted

sided then radical forms on more sub

N

Side

¥r •

But It will add to the side that

10.10 Anti Markovnikov Addition •

n

-

.

Br

,

.

Time 33:30

The Br radical adds to the less substituted carbon of the alkene , generating the more stable (more substituted) carbon radical

•

.

generates the more stable radical site as that will be a lower energy intermediate

energy transition state

{ thus a lower

.

The

.

carbon radical then abstracts an H to form the product 4 propagate the radical chain

.

10.10 Anti Markovnikov Addition

Time 33:51

Most of the radicals in solution at any given moment will be

•

typically

Br • radical SO combination of them is ,

Br

the termination step

RO

.

YY

.

Bro

any 2 radicals constitutes a termination step Always 6 termination steps since form 3 radicals

But the combination of

•

Termination :

RO NARO RO Br

F.

→

Br

→

# MRO

→

RO OR

RO

-

BIRO

→

Br

-

B r

→

.

Time 38:18

addition rxns Often

Recall that

!

Bryon Br BYE Br Br N Br Br → Bry # #→

Br

10.10 Addition Stereochemistry °

Br

Bro

-

.

form

chirality center

a new

If the substrate isn't chiral , then a racemic mixture will be obtained when one new chirality center Is formed

°

If the starting alkene contains a stereo center, then a mixture of diastercomers would be Obtained

.

Practice Time 38:50 What are the products of the following reaction? Br •

2 Chirality centers so 4 Major

I*

.

products

*

Bd

.

•

Br

a

=

•

Bd

.

a

•

a

10.11 Radical Polymerization

Br

a

=

÷

• =

E

=

Time 41:39

radical conditions are also frequently used to form polymers

•

Free

°

Recall that a

polymerization process joins together many small Units called monomers in a long chain

polymer long molecule with repeating motif ( monomers connecting) :

↳ instead

Of showing whole thing 1mm

part in brakets ↳

n

-

a I

Rogues rumor l l l

Ron NOR →

'

-

.

r :

ro

.

RO →

RO

Ron ,

-5

→

has 4 groups bonded to it

only need to know propagation step

roaring

form radical On most substituted side

) , we show small

way the polymer looks like depends on the monomer

.

oh

.

.

↳ remember alkene , *

.

Hof monomers : basic building block of polymers Always alkene

↳ the

Mechanism Ex : for asymmetrical alkene :

.

→

pony!!

" →

royyyor

.

10.11 Radical Polymerization Time 53:20 Radical polymerizations generally proceed through radical chain mechanism : •

**

°

The summation

of the propagation steps gives the overall netrin

10.11 Radical Polymerization •

Time 54:44

DONT NEED TO KNOW ( Just FUNZIES)

polymers with varying properties are obtained by polymerizing substituted ethylene derivatives :

Practice Time 58:17 What is the monomer?

(

* NO

H

'

"

"

Y"

y

A HH

K

HH

"

-

-

"

v

on HH

monomers always / what side do you form radical ( remember intermediate (more subside) alkenes

a HH

)int /

with 4 groups

Cl

¥

Cl

H

H

/

/

?

Cl

ROMI ,

→

,

Roti

10.13 Synthetic Utility of Halogenation °

Chlorination , to synthesize alkyl chlorides , is only useful when there Is only one possible product

at Karl °

Time 01:00:42

Brom nation

alkyl halide =

Slower

=

more selective

=

10.13 Synthetic Utility of Halogenation •

can be preformed when several Mono halogen at ton products are possible

Time 01:02:03

Overall the real advantage of halogenate on = functional Hing an otherwise unreactive alkane which can then be ,

,

converted to other useful functional groups

10.13 Review of Reactions •

Radical rxns =

.

Time 01:02:51

synthetically useful , 4 will be applied to multistep syntheses throughout course :

↳ know these 3 t

polymerization

Practice Time 01:03:13 Which sequence of reagents will complete the following synthetic sequence?

It

alkane

Bres alkyl NV

Bre

→

\

I

HV

Add HIM

O

halide

r

Br

Nao et →

I

NBS →

hv

C- 2

n

CH 10 FIN.

By I