Chapter 5

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5. An Overview of

Organic Reactions

Based on McMurry’s Organic Chemistry , 7th edition

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Why this chapter

!o understand organic and"or #ioche$istry, it

is necessary to %now&

'What occurs

'Why and how che$ica( reactions ta%e p(ace

We wi(( see how a reaction can #e descri#ed

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5.* +inds of Organic Reactions

n genera(, we (oo% at what occurs and try to (earn how it

happens

-o$$on patterns descri#e the changes

Addition reactions two $o(ecu(es co$#ine

/(i$ination reactions one $o(ecu(e sp(its into two

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1u#stitution parts fro$ two $o(ecu(es echange

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Rearrange$ent reactions a $o(ecu(e undergoes

changes in the way its ato$s are connected

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5.2 4ow Organic Reactions

Occur& Mechanis$s n a c(oc% the hands $ove #ut the mechanism #ehind

the face is what causes the $ove$ent

n an organic reaction, we see the transfor$ation that

has occurred. !he $echanis$ descri#es the steps#ehind the changes that we can o#serve

Reactions occur in defined steps that (ead fro$

reactant to product

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1teps in Mechanis$s

We c(assify the types of steps in a seuence

A step invo(ves either the for$ation or #rea%ing of a

cova(ent #ond

1teps can occur in individua((y or in co$#ination withother steps

When severa( steps occur at the sa$e ti$e they are

said to #e concerted

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!ypes of 1teps in Reaction

Mechanis$s Bond for$ation or #rea%age can #e sy$$etrica( or

unsy$etrica(

Symmetrical- ho$o(ytic

Unsymmetrical- hetero(ytic

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ndicating 1teps in Mechanis$s

-urved arrows indicate #rea%ing

and for$ing of #onds

Arrowheads with a 8ha(f9 head

:8fish'hoo%9; indicate ho$o(ytic

and ho$ogenic steps :ca((ed

<radica( processes’;

Arrowheads with a co$p(ete head

indicate hetero(ytic and

heterogenic steps :ca((ed <po(arprocesses’;

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5.) Radica( Reactions

>ot as co$$on as po(ar reactions

Radicals react to co$p(ete e(ectron octet of va(ence

she((

A radica( can #rea% a #ond in another $o(ecu(eand a#stract a partner with an e(ectron, giving

su#stitution in the origina( $o(ecu(e

A radica( can add to an a(%ene to give a new

radica(, causing an addition reaction

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!hree types of steps Initiation ho$o(ytic for$ation of two reactive species with

unpaired e(ectrons

/a$p(e for$ation of -( ato$s for$ -(2 and (ight Propagation reaction with $o(ecu(e to generate radica(

/a$p(e ' reaction of ch(orine ato$ with $ethane togive 4-( and -4)

.

Termination co$#ination of two radica(s to for$ a sta#(e

product& -4). ? -4). -4)-4)

1teps in Radica( 1u#stitution

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5.0 @o(ar Reactions

Mo(ecu(es can contain (oca( unsy$$etrica( e(ectron

distri#utions due to differences in e(ectronegativities

!his causes a partia( negative charge on an ato$ and a

co$pensating partia( positive charge on an adacent ato$

!he $ore e(ectronegative ato$ has the greater e(ectron

density

/(e$ents such as O, , >, -( $ore e(ectronegative than

car#on

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@o(ariCa#i(ity

Polarization is a change in e(ectron distri#ution as a

response to change in e(ectronic nature of the

surroundings

@o(ariCa#i(ity is the tendency to undergo po(ariCation

@o(ar reactions occur #etween regions of high

e(ectron density and regions of (ow e(ectron density

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Denera(iCed @o(ar Reactions

An electrophile, an e(ectron'poor species, co$#ines

with a nucleophile, an e(ectron'rich species

An e(ectrophi(e is a Eewis acid

A nuc(eophi(e is a Eewis #ase !he co$#ination is indicate with a curved arrow fro$

nuc(eophi(e to e(ectrophi(e

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5.5 An /a$p(e of a @o(ar Reaction&

Addition of 4Br to /thy(ene

4Br adds to the π part of -'- dou#(e #ond

!he π #ond is e(ectron'rich, a((owing it to function as

a nuc(eophi(e

4'Br is e(ectron deficient at the 4 since Br is $uch$ore e(ectronegative, $a%ing 4Br an e(ectrophi(e

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Mechanis$ of Addition of 4Br

to /thy(ene 4Br e(ectrophi(e is attac%ed #y

π e(ectrons of ethy(ene:nuc(eophi(e; to for$ acar#ocation inter$ediate and#ro$ide ion

Bro$ide adds to the positivecenter of the carbocation,which is an e(ectrophi(e,for$ing a -'Br σ #ond

!he resu(t is that ethy(ene and4Br co$#ine to for$

#ro$oethane A(( po(ar reactions occur #y

co$#ination of an e(ectron'rich site of a nuc(eophi(e andan e(ectron'deficient site of ane(ectrophi(e

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5.3 Fsing -urved Arrows in

@o(ar Reaction Mechanis$s -urved arrows are a way to %eep trac% of changes in

#onding in po(ar reaction

!he arrows trac% 8e(ectron $ove$ent9

/(ectrons a(ways $ove in pairs -harges change during the reaction

One curved arrow corresponds to one step in a

reaction $echanis$

!he arrow goes fro$ the nuc(eophi(ic reaction site tothe e(ectrophi(ic reaction site

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Ru(es for Fsing -urved Arrows

!he nuc(eophi(ic site can #e neutra( or negative(ycharged

!h ( t hi(i it # t (

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!he e(ectrophi(ic site can #e neutra( or

positive(y charged

Gon’t eceed the octet ru(e :or duet;

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5.7 Gescri#ing a Reaction& /ui(i#ria,

Rates, and /nergy -hanges

Reactions can go either forward or #ac%ward

to reach eui(i#riu$

!he $u(tip(ied concentrations of the products

divided #y the $u(tip(ied concentrations of thereactant is the equilibrium constant, Keq

/ach concentration is raised to the power of

its coefficient in the #a(anced euation.

aA + bB cC + d D

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Magnitudes of /ui(i#riu$

-onstants f the va(ue of K e is greater than *, this indicates that

at eui(i#riu$ $ost of the $ateria( is present as

products

f K e

is *=, then the concentration of the product is

ten ti$es that of the reactant

A va(ue of K e (ess than one indicates that at

eui(i#riu$ $ost of the $ateria( is present as thereactant

f K e is =.*=, then the concentration of the reactant

is ten ti$es that of the product

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ree /nergy and /ui(i#riu$

!he ratio of products to reactants is contro((ed #ytheir re(ative ibbs !ree energy

!his energy is re(eased on the favored side of aneui(i#riu$ reaction

!he change in Di##s free energy #etween productsand reacts is written as 8∆D9

f K e5 H *, energy is re(eased to the surroundings:e"ergonic reaction;

f K e5 I *, energy is a#sor#ed fro$ the surroundings:endergonic reaction;

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>u$eric Re(ationship of +e and ree

/nergy -hange !he standard free energy change at * at$ pressure

and 26 + is ∆DJ

!he re(ationship #etween free energy change and aneui(i#riu$ constant is&

∆DJ K ' RT (n K e5 where

R K *.67 ca(":+ $o(;

! K te$perature in +e(vin (n K e5 K natura( (ogarith$ of K e5

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5.6 Gescri#ing a Reaction& Bond

Gissociation /nergies Bond dissociation energy :D;& a$ount of energy reuired to

#rea% a given #ond to produce two radica( frag$ents when the

$o(ecu(e is in the gas phase at 25L -

!he energy is $ost(y deter$ined #y the type of #ond,

independent of the $o(ecu(e

!he -'4 #ond in $ethane reuires a net heat input of *=5

%ca("$o( to #e #ro%en at 25 J-. !a#(e 5.) (ists energies for $any #ond types

-hanges in #onds can #e used to ca(cu(ate net changes in heat

:/ntha(py K ∆4;

∑∑= formed)D(bonds- broken)D(bondsΔH

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5. Gescri#ing a Reaction& /nergy

Giagra$s and !ransition 1tates

!he highest energy

point in a reaction step

is ca((ed the transition

state

!he energy needed to

go fro$ reactant to

transition state is the

acti#ation energy

$∆D‡)

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)=

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)*

irst 1tep in Addition

n the addition of 4Brthe :conceptua(;transition'statestructure for the first

step !he π #ond #etween

car#ons #egins to#rea%

!he -4 #ond#egins to for$

!he 4Br #ond#egins to #rea%

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)2

5.*= Gescri#ing a Reaction&

nter$ediates f a reaction occurs in $ore

than one step, it $ustinvo(ve species that areneither the reactant nor thefina( product

!hese are ca((ed reactionintermediates or si$p(y8inter$ediates9

/ach step has its own freeenergy of activation

!he co$p(ete diagra$ forthe reaction shows the freeenergy changes associatedwith an inter$ediate

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))

5.** A -o$parison #etween Bio(ogica(

Reactions and Ea#oratory Reactions

Ea#oratory reactions usua((y

carried out in organic so(vent

Bio(ogica( reactions in

aueous $ediu$ inside ce((s

!hey are pro$oted #ycata(ysts that (ower the

activation #arrier

!he cata(ysts are usua((y

proteins, ca((ed enCy$es

/nCy$es provide ana(ternative $echanis$ that is

co$pati#(e with the

conditions of (ife

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Chapter 5