10hl.20.1 Types of Organic Reactions

8/18/2019 10hl.20.1 Types of Organic Reactions

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OPIC 20

ORGANIC

CHEMIS RY

20.1

TYPES OF ORGANIC REACTIONS


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ESSEN IAL IDEA

Key organic reaction types inc!"e n!ceop#iic

s!$stit!tion% eectrop#iic a""ition% eectrop#iic s!$stit!tion

an" re"o& reactions. Reaction 'ec#anis's (ary an" #ep in

!n"erstan"ing t#e "i))erent types o) reactions ta*ing pace.NAT+RE OF SCIENCE ,-.1/oo*ing )or tren"s an" "iscrepancies $y !n"erstan"ing "i))erent types

o) organic reactions an" t#eir 'ec#anis's% it is possi$e to synt#esie

ne co'po!n"s it# no(e properties #ic# can t#en $e !se" in

se(era appications. Organic reaction types )a into a n!'$er o)

"i))erent categories.NAT+RE OF SCIENCE ,3.1 an" 3.4

Coa$oration an" et#ica i'pications scientists #a(e coa$orate" to

or* on in(estigating t#e synt#esis o) ne pat#ays an" #a(e

consi"ere" t#e et#ica an" en(iron'enta i'pications o) a"opting

green c#e'istry.


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IN ERNA IONAL-

MINDEDNESS

5#at roe "oes green an"

s!staina$e c#e'istry% inreation to organic c#e'istry%

pay in a go$a conte&t6


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ORGANIC REAC IONS

Organic reactions are broadly organized

according to what happens (type of

reaction) and how it happens (themechanism of the reaction).

Many organic mechanisms describe the

reactants according to electrophilic andnucleophilic behavior.


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Nucleophiles are reactants that areelectron rich so they are very attracted to

electron deficient atoms. Nucleophiles have a lone pair of electrons

and may also carry a negative charge.They act as Lewis bases.

!O" O#" N$" %N

#

&lectrophiles are electron deficient andaccept electron pairs from a nucleophile.

They act as Lewis acids' " r and NO!.


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GUIDANCE

7e a$e to "i))erentiate $eteen

#o'oytic an" #eteroytic

)ission.


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GUIDANCE

Kno t#e "i))erence $eteen

c!ry arros an" )is#8#oo*s in

reaction 'ec#anis's.


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The curly arrow represents the motion of an

electron pair" with the tail showing where the

electrons come from and the full arrow showingwhere they are going.

The fishhoo* notation (half arrow on curve)

shows movement of one electron.

eterolytic fission + both electrons go to one

product and two oppositely charged ions are

produced

omolytic fission + the shared pair of electronsare split evenly between the ! atoms

Leaving group + the halogen that becomes

detached in the reaction.


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GUIDANCE

www.chemistr .about.com


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GUIDANCE

Typica con"itions an" reagents

o) a reactions s#o!" $e *non

s!c# as cataysts% re"!cing

agents% re)!&% etc. Speci)ic

te'perat!res nee" not $especi)ie".


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UNDERS ANDING/KEY

IDEA 20.1.A

SN1 represents a n!ceop#iic

!ni'oec!ar s!$stit!tion reaction

an" SN2 represents a n!ceop#iic$i'oec!ar s!$stit!tion reaction.

SN1 in(o(es a car$ocation

inter'e"iate. SN2 in(o(es a

concerte" reaction it# a transition

state.


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UNDERS ANDING/KEY

IDEA 20.1.B

For tertiary #aogenoa*anes% t#e

pre"o'inant 'ec#anis' is SN1 an")or pri'ary #aogenoa*anes% it is

SN2. 7ot# 'ec#anis's occ!r )or

secon"ary #aogenoa*anes.

N!ceop#iic S!$stit!tion Reactions


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APPLICA ION/SKILLS

7e a$e to "e"!ce t#e

'ec#anis' o) t#e n!ceop#iic

s!$stit!tion reactions o)

#aogenoa*anes it# a9!eo!s

so"i!' #y"ro&i"e in ter's o)SN1 an" SN2 'ec#anis's.



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NUCLEOPHILIC

SUBS I U ION

REAC IONS

The polarity in halogenoal*anes is due to the fact thatthe halogen atom is more electronegative than carbonand therefore it e,erts a stronger pull on the shared

electrons in the carbon#halogen bond. The halogen gains a partial negative charge and the

carbon gains a partial positive charge and is said to beelectron deficient.

This electron deficient carbon defines much of thereactivity of the halogenoal*anes.

Nucleophiles are reactants that are electron rich sothey are very attracted to electron deficient atoms.

Nucleophiles have a lone pair of electrons and may

also carry a negative charge' !O" O#" N$" %N#


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The nucleophiles are attracted to the electron deficientcarbon in the halogenoal*ane which leads to a reactionwhere substitution of the halogen occurs.

This reaction is called -ubstitution Nucleophilic andhas the shorthand notation of -N.

uring this reaction" the carbon#halogen bond brea*s and thehalogen is released as a halide ion.

/hen both of the shared electrons go to one product" it is*nown as heterolytic fission.

The halogen that becomes detached is sometimes referred toas the leaving group.

%!0r O#

1 %!0O r #


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PRIMARY

HALOGENOALKANES S

N

2

MECHANISM

2rimary halogenoal*anes have at least twohydrogens attached to the carbon of the carbon#halogen bond.

ecause the hydrogen atoms are so small" thecarbon is pretty open to an attac* from thenucleophile.

The rate of this one#step mechanism is dependent

upon the concentration of both thehalogenoal*ane and the hydro,ide ion" it is *nownas a bimolecular reaction.

This mechanism is fully described as -N!'substitution nucleophilic bimolecular.

%$%l O# 1 %$O %l

#


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This is a one#step concerted reactionwith an unstable transition state.

Note the nucleophile attac*s the

electrophilic carbon atom on the opposite

side of the leaving group which causes

an inversion in the arrangement of the

atoms around the carbon atom. (Li*e an

umbrella blowing inside out)

www.iversion.cm.ute,as.edu


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The -N! reaction is stereospecific

because the three#dimensional

arrangement of the reactants determinesthe three#dimensional configuration of

the products.

This happens because bond formationcomes before bond cleavage in the

transition state and the stereochemistry

is not lost.


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ER IARY

HALOGENOALKANES:

S

N

1 MECHANISM

Tertiary halogenoal*anes have three

al*yl groups attached to the carbon of the

carbon#halogen bond. These bul*y al*yl groups ma*e it difficult

for an incoming group to attac* the

carbon atom causing steric hindrance.


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GUIDANCE

7e a$e to 'a*e re)erence to

#eteroytic )ission in SN

1

reactions.



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4ou should be able to write rate e5uations for

the rate#determining step in both reaction

mechanisms using your *nowledge of *inetics.


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UNDERS ANDING/KEY

IDEA 20.1.CT#e rate "eter'ining step ,so

step in an SN1 reaction "epen"s

ony on t#e concentration o) t#e#aogenoa*ane% rate :

*;#aogenoa*ane


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UNDERS ANDING/KEY

IDEA 20.1.D

SN2 reactions are $est con"!cte"

!sing aprotic% poar so(ents an"

SN1 reactions are $est con"!cte"

!sing protic% poar so(ents.



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APPLICA ION/SKILLS

7e a$e to o!tine t#e "i))erence

$eteen protic an" aprotic

so(ents.



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S

N

2 prefer p!"#r$

#pr!%&' !"(e)%

6protic solvents are those which are not

able to form hydrogen bonds as they do

not contain O or N bonds. They may have strong dipoles.

They solvate the metal ion rather than the

nucleophile.

The unsolvated bare nucleophile has ahigher energy state and increases the rate of

reaction.

&,amples are propanone" ethanenitrile and

ethyl ethanoate.


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S

N

1 prefer p!"#r$ pr!%&'

!"(e)%

2rotic solvents are those which are able

to form hydrogen bonds and they do

contain O or N bonds. They stabilize the carbocation intermediate.

They do this by solvation involving ion#dipole

interactions.

&,amples are water" alcohols and carbo,ylic

acids.


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APPLICA ION/SKILLS

7e a$e to e&pain #y #y"ro&i"e is

a $etter n!ceop#ie t#an ater.



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HYDRO*IDE + A BE ER

NUCLEOPHILE

N!ceop#ies are reactants t#at areeectron ric# so t#ey are (ery

attracte" to eectron "e)icient ato's. N!ceop#ies #a(e a one pair o)

eectrons an" 'ay aso carry anegati(e c#arge= >2O% O>

8% N>-% CN8

T#e #y"ro&i"e ion is a strongern!ceop#ie t#an ater $eca!se itcarries a negati(e c#arge.


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APPLICA ION/SKILLS

7e a$e to e&pain #o t#e rate

"epen"s on t#e i"entity o) t#e

#aogen ,ie t#e ea(ing gro!p%

#et#er t#e #aogenoa*ane is

pri'ary% secon"ary or tertiaryan" t#e c#oice o) so(ent.



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IDEN I Y O, HE

HALOGEN

The identity of the halogen influences the rate ofthe nucleophilic substitution.

There are however two opposing factors. 3. The polarity of the carbon#halogen bond

&lectronegativity decreases down 7roup 8 The carbon#halogen bond becomes progressively less

electron deficient so it is less vulnerable to nucleophilicattac*.

/e would e,pect fluoroal*ane to be the most reactiveand iodoal*ane to be the least.

O/&9&:;.


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!. -trength of the carbon#halogen bond ond energy data shows that the carbon#halogen

bond decreases in strength from fluorine to

iodine.

-ince nucleophilic substitution involves the

brea*ing of this bond" one would e,pect that the

iodoal*ane to be the most reactive and

fluoroal*ane to be the least reactive. :eaction rate data indicates that the carbon#

halogen bond strength dominates the outcome.


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RA E O, REAC ION

The rate of nucleophilic substitution

reactions in halogenoal*anes depends

on the class of the halogenoal*ane.


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UNDERS ANDING/KEY

IDEA 20.1.E

An eectrop#ie is an eectron8

"e)icient species t#at can accept

eectron pairs )ro' a n!ceop#ie.Eectrop#ies are /eis aci"s

,eectron pair acceptors.

Eectrop#iic A""ition Reactions


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APPLICA ION/SKILLS

7e a$e to "e"!ce t#e

'ec#anis' o) t#e eectrop#iic

a""ition reactions o) a*enes

it# #aogens?inter#aogens an"

#y"rogen #ai"es.,Inter#aogens are co'po!n"s 'a"e !p o) to

or 'ore #aogens.

Eectrop#iic A""ition Reactions


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ELEC ROPHILIC

ADDI ION REAC IONS

6l*enes are unsaturated molecules and

readily undergo addition reactions.

6n addition reaction occurs when tworeactants combine to form a single

product.

This type of reaction is characteristic incompounds containing double or triple

bonds (unsaturated compounds).


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The carbon atoms of a double bond are

sp! hybridized and have a trigonal planar

shape with bond angles of 3!> degrees. This is a fairly open structure ma*ing it

relatively easy for groups to attac*.

The bond is composed of a sigma bondand a pi bond which has an area of

electron density above and below the

plane of the bond a,is.


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ecause the electrons in the pi bond are

less closely associated with the nuclei" it

is a wea*er bond than the sigma bondand so brea*s more easily during the

addition reactions.

ecause the pi bond is and area ofelectron density" it is very attractive to

electrophiles (species that are electron

deficient).


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:eactions between electron deficient

reagents and al*enes are *nown as

electrophilic addition reactions. There are four types of these

mechanisms that we will study.

6l*enes halogens 6l*enes hydrogen halides

6l*enes interhalogens

?nsymmetrical al*enes hydrogen halides


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ALKENES HALOGENS

E%e)e r!&)e

/hen ethene gas is bubbled through

bromine" the brown color disappears as it

forms 3"!#dibromoethane.

:emember that the color change shows

presence of unsaturation.

http'@@www.bbc.co.u*@staticarchive


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The mechanism is as follows'

romine" a non#polar molecule" becomes polarized

as it approaches the electron rich area of the

al*ene. The electron rich area repels the electrons on the

bromine molecule causing an area of positive

charge and an area of negative charge.


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The bromine atom nearest the al*eneAs double

bond gains a partial positive charge and acts

as the electrophile.

The bromine molecule splits heterolytically

forming r and r # and the initial attac* on the

ethene in which the pi bond brea*s is carried

out by r

. This first step is slow forming an unstable

positive carbocation intermediate.


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This carbocation then reacts rapidly with

the negative bromide ion forming the

product.


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ALKENES HYDROGEN HALIDES

E%e)e 34r!5e) r!&4e

The reaction occurs by a similar reaction

as al*enes halogens.

r is a polar molecule that undergoesheterolytic fission to form and r #.

The electrophile" " attac*s the al*eneAs

double bond. The unstable positive carbocation

intermediate then reacts readily with the

r #

to form the addition product.


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6 piece of evidence that supports this

mechanism is that the reaction if favored by a

polar solvent that facilitates the production ofions from heterolytic fission.

< = r = %l


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ALKENES IN ERHALOGENS

E%e)e BrC"

6n interhalogen is a compound made up

of two halogens.

%hlorine is more electronegative thanbromine so r%l is polarized so that r

has a partial positive charge and is the

electrophile and %l has the partialnegative charge and will react with the

carbocation.

raw the reaction mechanism.


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UNSYMME RIC

ADDI ION

Pr!pe)e 34r!5e) r!&4e

/hen unsymmetric al*enes react" there

are two different products that can form

resulting from the two possible pathways

that come from the two possible

carbocations that can form.

The difference comes from whether the

attac*ing electrophile bonds to carbon 3

in the double bond or carbon !.

The answer comes from which pathway

will give the most stable carbocation.


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SAMPLE PROBLEM

/rite the names and structures for the

two possible products of the addition of

the interhalogen r%l to propene. /hich is li*ely to be the maBor productC

&,plain.


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S!"8%&!)


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UNDERS ANDING/KEY

IDEA 20.1.G

7enene is t#e si'pest aro'atic

#y"rocar$on co'po!n" ,or arene an"

#as a "eocaie" str!ct!re o) $on"saro!n" its ring. Eac# car$on to car$on

$on" #as a $on" or"er o) 1.4. 7enene is

s!scepti$e to attac* $y eectrop#ies.

Eectrop#iic S!$stit!tion Reactions


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SUBS I U ION

REAC IONS

BEN9ENE

enzene is highly unsaturated due to its three

double bonds" but it does not behave li*e

al*enes.

The benzene ring favors substitution reactions

over addition reactions.

enzene is very attractive to electrophiles

because of its high electron density. The delocalized cloud of pi electrons see*s

electron#deficient species and forms a new bond

when a hydrogen is lost.


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APPLICA ION/SKILLS

7e a$e to "e"!ce t#e 'ec#anis' o)

t#e nitration ,eectrop#iic

s!$stit!tion reaction o) $enene,!sing a 'i&t!re o) concentrate"

nitric aci" an" s!)!ric aci".

Eectrop#iic S!$stit!tion Reactions


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NI RA ION O, BEN9ENE

The nitration of benzene is the

substitution of an DE by DNO!E to form

nitrobenzene" %F0NO!. The overall reaction is

%FF NO$ %F0NO! !O

The electrophile is the nitronium ion" NO!.

This ion is generated by using a Dnitrating

mi,tureE which is mi,ture of concentrated nitric

and sulfuric acid at 0>G

%.

%onc !-OH

0>G%


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The sulfuric acid protonates the nitric

acid which then loses a molecule of

water to produce NO!

.


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NO! is a strong electrophile and reacts with the

pi electrons of the benzene ring to form the

carbocation intermediate.

Loss of a proton from this leads to reformation of

the arene ring in the product nitrobenzene which

appears as a yellow oil. The released reacts with the base -OH

# to

reform sulfuric acid.


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UNDERS ANDING/KEY

IDEA 20.1.H

Car$o&yic aci"s can $e re"!ce" to

pri'ary aco#os ,(ia t#e a"e#y"e.

Ketones can $e re"!ce" to secon"aryaco#os. Typica re"!cing agents are

it#i!' a!'ini!' #y"ri"e ,!se" to

re"!ce car$o&yic aci"s an" so"i!'

$oro#y"ri"e.

Re"!ction Reactions


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APPLICA ION/SKILLS

7e a$e to rite re"!ction reactions

o) car$ony containing co'po!n"s=

a"e#y"es an" *etones to pri'aryan" secon"ary aco#os an"

car$o&yic aci"s to a"e#y"es% !sing

s!ita$e re"!cing agents.

Re"!ction Reactions


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oth reducing agents produce the hydride

ion" #" which acts as a nucleophile on the

electron deficient carbonyl carbon. IJ represents reduction Bust as IOJ

represents o,idation.

NaH is the safer reagent but is notreactive enough to reduce carbo,ylic acids

so Li6lH must be used in this case.


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Re48'%&!) !f # Ke%!)e


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Re48'%&!) !f #

C#r!3"&' A'&4


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REDUC ION O,

NI ROBEN9ENE

Nitrobenzene (%F0NO!) can be

converted to phenylamine"

%F0N! in a two step reduction

process.


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S%ep 1

%F0NO! is reacted with a mi,ture of tin andconcentrated %l.

The reaction mi,ture is heated under reflu, in a

boiling water bath.

The product %F0N$" phenylammonium ions" is

protonated because of the acidic conditions.


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S%ep 2

%F0N$ is reacted with sodium

hydro,ide to remove and form the

product phenylamine" %F0N!.


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SUMMARY O,

REAC ION

MECHANISMS


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C&%#%&!)

3H. 2rint.

Most of the information found in this power point comes directly

from this te,tboo*. 2hotos and graphs are copied directly from the

te,t.

The power point has been made to directly complement the igher

Level %hemistry te,tboo* by %atrin and rown and is used for

direct instructional purposes only.

Documents

10hl.20.1 Types of Organic Reactions