section 2 - alkenes and halogenoalkanes powerpoint

7/26/2019 Section 2 - Alkenes and Halogenoalkanes Powerpoint

1/102

Bonding in methane,

ethane and etheneand bonds

AS Chemistry


2/102

Learning Objectives

Candidates should be able to:

describe covalent bonding in terms of orbital overlap,giving and bonds.

explain the shape of, and bond angles in, ethane andethene molecules in terms of and bonds.


3/102

Starter activity


4/102

Alkenes

pent-2-ene CH3

CH=CHCH2

CH3

hex-3-ene CH3CH2CH=CHCH3

2,3-dimethylpent-2-ene

cyclopenta-1,3-diene

3-ethylhept-1-ene CH2=CHCH2CHCH2CH3!CH2CH2CH3


5/102

Hybridisation of orbitals

"he electronic configuration of a carbon atom iss!!s!!"!

1 1s

22s

2p


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H#B$%&%SA'%O( O) O$B%'ALS#f you provide a bit of energy you can promote lift!one of the s electrons into a p orbital. "heconfiguration is no$ s!!s!"*

1 1s

2

2s

2p

"he extra energy released $hen the bonds form more thancompensates for the initial input.


7/102

Hybridisation of orbitals in alkanes"he four orbitals an s and three p%s! combine or

H#B$%&%S+to give four ne orbitals. &ll four orbitals aree-uivalent.

'ecause one s and three p orbitals are used, it is called s"*

hybridisation.

2s22p2 2s12p3 4 x sp3


8/102

sp3orbitals

#n AL/A(+S, the four sp3orbitals repel each otherinto a tetrahedralarrangement.

Hybridisation of orbitals in alkanes
http://winter.group.shef.ac.uk/orbitron/AO-hybrids/sp3/index.htmlhttp://winter.group.shef.ac.uk/orbitron/AO-hybrids/sp3/index.htmlhttp://winter.group.shef.ac.uk/orbitron/AO-hybrids/sp3/index.htmlhttp://winter.group.shef.ac.uk/orbitron/AO-hybrids/sp3/index.htmlhttp://winter.group.shef.ac.uk/orbitron/AO-hybrids/sp3/index.htmlhttp://winter.group.shef.ac.uk/orbitron/AO-hybrids/sp3/index.htmlhttp://winter.group.shef.ac.uk/orbitron/AO-hybrids/sp3/index.html


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Bonding in methane


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Bonding in ethane


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Bonding in ethene

&lternatively, only three orbitals an s and t$o p%s!combine or H#B$%&%S+ to give three ne orbitals. &llthree orbitals are e-uivalent. "he remaining 2p orbital isunchanged.

2s22p2 2s12p3 3 x sp2 2p


12/102

s"!hybrids

0hat about ethene1


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2 bonds


14/102

3eometric %somerism

AS Chemistry


15/102

Learning Objectives


describe cis-trans isomerism in al(enes, and explainits origin in terms of restricted rotation due to the

presence of ) bonds.

deduce the possible isomers for an organicmolecule of (no$n molecular formula.

identify cis-trans isomerism in a molecule of givenstructural formula.


16/102

Starter activity


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ISOMERISM

STRUCTURAL ISOMERISMSTEREOISOMERISM

GEOMETRIC

ISOMERISM OPTICAL ISOMERISM

0hat is stereoisomerism1

#n stereoisomerism, the atoms ma(ing up the isomersare *oined up in the same order, but still manage to havea different arrangement in space


18/102

3eometric %somerism1


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3+O4+'$%C %SO4+$%S4$+S'$%C'+& $O'A'%O( O) C5C BO(&S

Single covalent bonds can easily rotate. 0hat a""earsto be a different structure in an alkane is not. &ue tothe ay structures are ritten out, they are thesame.

ALL 'H+S+ S'$6C'6$+S A$+ 'H+ SA4+ B+CA6S+ C2C BO(&S HA7+ 8)$++9$O'A'%O(

Animation doesnt

work in old

versions of

Powerpoint


20/102



21/102

3eometric isomers of but2!2ene


22/102



23/102

3+O4+'$%C %SO4+$%S4Ho to tell if it e;ists

'odifferentatoms


24/102

3+O4+'$%C %SO4+$%S4%somerism in butene

'here are * structural isomers of C=H>that are alkenes?. Of theseO(L# O(+ e;hibits geometrical isomerism.

B6'22+(+ !24+'H#L@$O@+(+transB6'2!2+(+cisB6'2!2+(+

*#O6 CA( 3+' AL/A(+S 0%'H )O$46LA C=H> %) 'H+ CA$BO( A'O4S A$+ %( A$%(3


25/102

Summary

"o get geometric isomers you must have+

restricted rotation involving a carbon-carbon double

bond for &-level purposes!

t$o different groups on the left-hand end of the bondand t$o different groups on the right-hand end. #tdoesnt matter $hether the left-hand groups are thesame as the right-hand ones or not.


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'he effect of geometric isomerism on"hysical "ro"erties

isomer melting "ointC

boiling "ointC

cis -/ DE

trans 2FE 0

ou $ill notice that+the trans isomer has the higher melting pointthe cis isomer has the higher boiling point.


27/102

0hy is the boiling "oint of the cisisomers higher1

"he difference bet$een the t$o is that the cis isomer is

a polar molecule $hereas the trans isomer is non-polar.


28/102

0hy is the melting "oint of the cisisomers loer1

#n order for the intermolecular forces to $or( $ell, themolecules must be able to pac( together efficiently in the

solid.

"rans isomers pac( better than cis isomers. "he shapeof the cis isomer doesnt pac( as $ell as the straighter

shape of the trans isomer.


29/102

O"tical %somerism

AS Chemistry


30/102

Learning Objectives


explain $hat is meant by a chiral centre and thatsuch a centre gives rise to optical isomerism.

deduce the possible isomers for an organicmolecule of (no$n molecular formula.

identify chiral centres in a molecule of givenstructural formula.


31/102

Starter activity


32/102

O"tical isomerism

5hen four different atoms or groups are attached to a

carbon atom, the molecules can exist in t$o isomeric forms(no$n as o"tical isomers. "hese are non-superimposablemirror images.

Chiral centre

Chiral molecule


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O"tical %somerism0hat is a non2su"erim"osable mirror image1

Animation doesnt

work in old

versions of

Powerpoint


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O"tical isomerism

&mino acids the building bloc(s of proteins! are opticallyactive. "hey affect plane polarised light differently.


35/102

Butan2!2ol


36/102

O"tical %somerism'he "olarimeter

%f the light a""ears to have turned to the right turned to the left &+'$O$O'A'O$# LA+7O$O'A'O$#

A Light source "roduces light vibrating in all directionsB @olarising filter only allos through light vibrating in one directionC @lane "olarised light "asses through sam"le& %f substance is o"tically active it rotates the "lane "olarised light+ Analysing filter is turned so that light reaches a ma;imum

) &irection of rotation is measured coming toards the observer

A B

C DE

F


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+nantiomers G ho do they differ1

sually have the same chemical and

physical properties 6 but behavedifferently in presence of otherchiral compounds.


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+nantiomers G ho do they differ1


39/102

'#@+S O) %SO4+$%S4

Occurs due to the restrictedrotation of C5C doublebonds... to forms 2 C%S and

'$A(S

S'$6C'6$AL %SO4+$%S4

S'+$+O%SO4+$%S4

3+O4+'$%CAL %SO4+$%S4

O@'%CAL %SO4+$%S4

CHA%( %SO4+$%S4

Same molecular formula butdifferent structuralformulae

Occurs hen molecules have achiral centre. 3et to non2su"erim"osable mirror images.

Same molecularformula but atomsoccu"y different"ositions in s"ace.

@OS%'%O( %SO4+$%S4

)6(C'%O(AL3$O6@ %SO4+$%S4


40/102

+lectro"hilicAddition to Alkenes

AS Chemistry


41/102

Learning Objectives


describe the mechanism of electrophilic addition in

al(enes, using bromine7ethene as an example.

describe the chemistry of al(enes as exemplified,$here relevant, by the follo$ing reactions of ethene+

addition of hydrogen, steam, hydrogen halides andhalogens.


42/102

Starter activity


43/102

CH2=CH2 + Br2 CH2BrCH2Br

+lectro"hilic addition


44/102

+lectro"hilic addition

CH2=CH2 + Br2 CH2BrCH2Br

bromine ith ethene

hydrogen bromide ith ethene

CH2=CH2 + HBr CH3CH2Br

bromoethane

1,2-dibromoethane

+l t hili dditi h i


45/102

Br

Br

Br

Br

+lectro"hilic addition mechanism

H

H H

H

CC

+

-

H

H H

HCC

Br

+

Br-

carbocation

H

H H

HCC

Br Br1,2-dibromoethane

bromine ith ethene

+l t hili dditi h i


46/102

+lectro"hilic addition mechanism

H

H H

H

CC

H

H H

HCC

H

+

carbocation

H

H H

HCC

Br H

bromoethane

hydrogen bromide ith ethene

-

+

Br

H

Br-


47/102

+lectron flo during electro"hilic addition


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+6A'%O( '+4@+$A'6$+OC

@$+SS6$+ CA'AL#S' @HAS+ (O'+S

hydrogen CH2=CH28 H2CH3CH3 91:/;inely divided

nic(el on

support

material

CH3CH2>H

33/ ?@Aa

Ahosphoric B!

acid H3A>0!

adsorbed ontothe surface of

silica.


49/102

Addition to unsymmetrical alkenes

+lectro"hilic addition to "ro"ene

2-bromopropane

1-bromopropane

Additi t t i l lk


50/102

%n the electro"hilic addition to alkenes the major"roduct is formed via the more stable carbocationcarbonium ion

least stable most stablemethyl I "rimary I secondary ! I tertiary *


Additi t t i l lk


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PATH A

PATH B

4AJO$ @$O&6C'

@$%4A$#CA$BOCA'%O(

S+CO(&A$#CA$BOCA'%O(

4%(O$ @$O&6C'



52/102

@olymerisation

AS Chemistry


53/102

Learning Objectives

Candidates should be able to:describe the chemistry of al(enes including

polymerisation.

describe the characteristics of additionpolymerisation as exemplified by polyethene! andABC.

ecogniFe the difficulty of the disposal ofpolyal(ene!s, i.e. non-biodegradability and harmfulcombustion products.


54/102

Starter activity

@olyethene


55/102

@olyethene

'em"erature: about 2//GC

@ressure:about 2/// atmospheres

%nitiator: often a small amount of oxygenas an impurity

Conditions

)ree radical addition


56/102

)ree radical addition

#nitiation

Aropagation

"ermination

Aropagation

L&@+ or H&@+


57/102

L&@+ or H&@+

L&@+ or H&@+


58/102

L&@+ or H&@+

;reeFer bags, $ater pipes, $ire and cable insulation,extrusion coating

and$ich bags, cling $rap, car covers, sDueeFe bottles,liners for tan(s and ponds, moisture barriers in

construction

@olymerisation of alkenes


59/102

@olymerisation of alkenes

+'H+(+ @OL#+'H+(+

'+'$A)L6O$O+'H+(+@OL#'+'$A)L6O$O+'H+(+

@')+ K'eflon

@$O@+(+ @OL#@$O@+(+

CHLO$O+'H+(+@OL#CHLO$O+'H+(+@OL#7%(#LCHLO$%&+ @7C

&is"osal of "olymers


60/102

4ethod Comments

Eandfill Imissions to the atmosphere and $atervermin unsightly. Can ma(e use of old

Duarries.

#ncineration aves on landfill sites and produces

energy. @ay also release toxic and

greenhouse gases.

ecycling high cost of collection and re-

processing.;eedstoc(

recycling

se the $aste for the production of

useful organic compounds. e$

technology can convert $aste into

hydrocarbons $hich can then be turned

&is"osal of "olymers


61/102

O;idation ofalkenes

AS Chemistry


62/102

Learning Objectives

Candidates should be able to describe the o;idationof alkenes by:

cold, dilute, acidified manganateB##! ions to formthe diol, and

hot, concentrated, acidified manganateB##! ionsleading to the rupture of the carbon-to-carbondouble bond in order to determine the position of

al(ene lin(ages in larger molecules.


63/102

Starter activity

O;idation of alkenes


64/102


#n the presence of diluteacidified or al(aline! potassiummanganate B##!.

&l(enes react readily at room temperature i.e. in the

cold!."he purple colour disappears and a diol is formed.

CH2=CH2 8 H2> 8 J>K H>CH2CH2>H

ethane 6 1,2-diol



65/102


)ragment @roduct

=CH2C>2

-CH=

&ldehyde carboxylic acid

2C=

Letone

#n the presence of a hot, concentrated solution of

acidified potassium manganate B##!, any diol formed issplit into t$o fragments $hich are oxidiFed further tocarbon dioxide, a (etone or a carboxylic acid.



66/102

O;idation of alkenes1. CH2=CH2

2. CH3CH=CH2

3. CH3!2C=CH2

2 products 6 bothcontain (etone

2 products 6 one contains2 (etone groups and onecontains 2 acid groups.

1 product only


67/102

Halogenoalkanes

AS Chemistry


68/102

Learning Objectives

Candidates should be able to recall the chemistry ofhalogenoal(anes as exemplified by the follo$ingnucleophilic substitution reactions of bromoethane+

hydrolysis

formation of nitriles

formation of primary amines by reaction $ith

ammonia.


69/102

Starter activity


70/102

a. CHCl3 trichloromethaneb. CH3CHClCH32-chloropropane

c. C;3CCl3 1,1,1-trichloro-2,2,2-trifluoroethane

(aming Halogenoalkanes

F

F

Cl

Cl

F Cl


71/102

@hysical @ro"erties

a. 1-chloropropane is polar and has permanent dipole-dipole intermolecular forces that are stronger thanthe temporary dipole-induced dipole forces in non-polar butane.

b. 1-chloropropane is polar and has permanent dipole-dipole intermolecular forces that are stronger than

the temporary dipole-induced dipole forces in non-polar butane.


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(ucleo"hilic substitution

negotiate cleveralp or

cadet tart

eat given

enticed if

chenille soup

had lie

stubs tuition

electronegativepolar

attracted

negative

deficient

nucleophiles

halide

substitution


73/102


"his is (no$n as an S(!reaction.

Sstands for substitution,

(for nucleophilic, and

!because the initial stage ofthe reaction involves t$o species.

( l "hili b tit ti n m h ni m


74/102

(ucleo"hilic substitution 2 mechanism

A(%4A'%O( SHO0%(3 'H+ S(! 4+CHA(%S4

&ttac( by nucleophile is to the bac( of the molecule a$ay from the negatively charged halogen atom.

$ate of reaction


75/102

$ate of reaction

ou may expect the fluoroal(ane to react more Duic(ly as the C-; bond is themost polar and therefore more susceptible to attac( by nucleophiles. Ho$ever,the C-; bond is the strongest. & nucleophile may be more attracted morestrongly to the carbon atom but, unless it forms a stronger bond to carbon, it$ill not displace the halogen.

&ctually the reaction $ith the iodoal(ane is the most rapid. "his suggests thatthe strength of the C-M bond is more important than its polarity. ote thatthe C-# bond is not polar. Ho$ever, it is easily polarisable.

Halogen ) Cl Br %

Ilectronegativity 0./ 3./ 2. 2.:

'ond strength C-M!

(N mol-100 33 2O? 23

4easuring the rate of reaction


76/102

+;"eriment

5ater is a poor nucleophile but it can slo$ly displace halideions

C!HFBrl M H!Ol C!HFOHl M HMa- M BrNa-

#f aDueous silver nitrate is sha(en $ith a halogenoal(anethey are immiscible! the displaced halide combines $ith a

silver ion to form a precipitate of a silver halide. "he$ea(er the C-M bond the Duic(er the precipitate appears.

4easuring the rate of reaction



77/102

hydro;ide ion ith bromoethane

ethanolCH3CH2Br + OH

- CH3CH2OH+ Br-

aDueous!


0ater ith bromoethane

ethanolCH3CH2Br + H2O CH3CH2OH+ HBraDueous!

"his is a slo$er reaction 6 $ater is not such a good nucleophile.

$arm

$arm

(ucleo"hilic substitution mechanism


78/102

M 2CH3

H

BrC

H

-OH

CH3

H

OHC

H Br-

hydro;ide ion ith bromoethane


ethanol



79/102

ater ith bromoethane


ethanol

+ -CH3

H

BrC

H

Br-

H

CH3

H

OHC

H

+

CH3

H

OHC

HHBr

H2O



80/102

propanenitrile

CH3CH2Br + CN-(ethanol) CH3CH2CN+ Br

-

cyanide ion ith bromoethane

ammonia ith bromoethane

CH3CH2Br + NH3(ethanol) CH3CH2NH22 + NH4+Br-


aminoethane

CH3CH2Br + NH3(ethanol) CH3CH2NH2 + HBr

reflux

Heat 7

pressure

Heat 7

pressure



81/102

M 2CH3

H

BrC

H

CN

-

CH3

H

CNC

H Br-

cyanide ion ith bromoethane


propanenitrile



82/102

ammonia ith bromoethane


aminoethane

+ -CH3

H

BrC

H

Br-

H

CH3

H

NH2C

H

+

CH3

H

NH2C

H

NH3

H NH3+Br -

NH3

@ast "a"er -uestion


83/102

@ast "a"er -uestion

Cl2

U.V. /!nl"#ht$thanol"%&CNre'!

Br2

U.V. /!nl"#ht


84/102

Substitution vs.+limination

AS Chemistry

L i Obj ti


85/102

Learning Objectives

Candidates should be able to+recall the chemistry of halogenoal(anes as

exemplified by the elimination of hydrogen bromidefrom 2-bromopropane.

describe the mechanism of nucleophilic substitutionby both 1 and 2 mechanisms! in halogenoal(anes.

S i i


86/102

Starter activity

'y"e of @osition of +;am"le


87/102

y" fhalogenoalkane

fhalogeno2 grou"

+ m"

primaryat end of chain+ bromoethane

secondary in middle of chain+ 2-bromopropane

tertiary attached to a carbon atom $hichcarries no H atoms+2-bromo-2-methylpropane

S( G tertiary halogenoalkanes


88/102

S( tert ary halogenoalkanes

ucleophilic attac( at the bac( of the molecule is

hindered by bul(y CH3 groups. "ertiary carbocation isstabilised by electron donating effect of CH3groups.

S S ! 1


89/102

S( or S(! 1

Halogenoalkane 4echanism

Arimary 2

econdary 1 and 2

"ertiary 1

+limination


90/102

m

ou need to be a$are that the hydroxide ion can act as a

strong base as $ell as a nucleophile.

&n alternative reaction can ta(e place in $hich H'r isremoved and an al(ene is formed. "his is (no$n as

elimination.

CH3CH2'r 8 a>H CH2=CH2 8 a'r 8 H2>

+limination of H from haloalkanes


91/102

+limination of HBr from !2bromo"ro"ane

CH3

H H

H

CC

OH-

CH3

H H

HCC

Br H

propene

H OHBr -

CH3CHBrCH3 + OH- CH3CH=CH2+ H2O + Br

-

(in ethanol)

acting as a base

+ m f f m

Substitution or +limination1


92/102

2

elimination

+ OH-

*CH=CH2+ H2O + -

ethanol!

nucleophilic substitution alcohol

+ OH-

*CH3CH2OH+ Br-

aDueous!

*CH2CH2

al(ene

hydroxide acts as a base

hydroxide acts as a nucleo"hile


93/102

AS Chemistry

@ros and Cons

Learning Objectives


94/102

Learning ObjectivesCandidates should be able to+

interpret the different reactivities ofhalogenoal(anes e.g. C;Cs anaesthetics flameretardants plastics $ith particular reference tohydrolysis and to the relative strengths of the C-Hal

bondsexplain the uses of fluoroal(anes and

hydrofluorooal(anes in terms of their relativechemical inertness

recognise the concern about the effect ofchlorofluoroal(anes on the oFone layer.

St rt r ctivit


95/102

Starter activity

Chlorofluorocarbons C)Cs


96/102

Chlorofluorocarbons 2 C)Cs

.Aroperties+

on-flammable

Eo$ toxicity

nreactive

EiDuefy easily $hen compressed


97/102

6ses

$efrigerants

@ro"ellants for aerosols

Solvents including dry2cleaning&egreasers

'he oone layer


98/102

'he oone layer

atural oFone layer


99/102

atural oFone layer


100/102


101/102

$e"lacements

Hydrochlorofluorocarbons, HC)Cs: shorter life in theatmosphere.

Hydrofluorocarbons, H)Cs: don%t contain chlorine soFero affect on oFone layer.

Hydrocarbons: Fero effect on oFone layer butflammable and lead to photochemical smog.

C. ,h " BCF #oo at et"n#!"h"n# re0


102/102

1he reen%e o a brom"ne %oner 'ame retar"n# !al"t"e on thero!%t.

1he h"#h temerat!re "n re brea5 th" %omo!n o6n7 ro!%"n# reera"%al !%h a Br8. 1hee rea%t 6"th other ree ra"%al ro!%e!r"n# %omb!t"on7 !en%h"n# the 'ame.

section 2 - alkenes and halogenoalkanes powerpoint

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