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Bonding in methane,
ethane and etheneand bonds
AS Chemistry
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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.
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Starter activity
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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
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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 $hen the bonds form more thancompensates for the initial input.
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Hybridisation of orbitals in alkanes"he four orbitals an s and three p%s! combine or
H#B$%&%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
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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.html7/26/2019 Section 2 - Alkenes and Halogenoalkanes Powerpoint
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Bonding in methane
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Bonding in ethane
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Bonding in ethene
<ernatively, 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
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s"!hybrids
0hat about ethene1
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2 bonds
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3eometric %somerism
AS Chemistry
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Learning Objectives
Candidates should be able to:
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$n molecular formula.
identify cis-trans isomerism in a molecule of givenstructural formula.
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Starter activity
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ISOMERISM
STRUCTURAL ISOMERISMSTEREOISOMERISM
GEOMETRIC
ISOMERISM OPTICAL ISOMERISM
0hat is stereoisomerism1
#n stereoisomerism, the atoms ma(ing up the isomersare *oined up in the same order, but still manage to havea different arrangement in space
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3eometric %somerism1
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3+O4+'$%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$O'A'%O(
Animation doesnt
work in old
versions of
Powerpoint
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3eometric %somerism1
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3eometric isomers of but2!2ene
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3eometric %somerism1
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3+O4+'$%C %SO4+$%S4Ho to tell if it e;ists
'odifferentatoms
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3+O4+'$%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
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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 $ill notice that+the trans isomer has the higher melting pointthe cis isomer has the higher boiling point.
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0hy is the boiling "oint of the cisisomers higher1
"he difference bet$een the t$o is that the cis isomer is
a polar molecule $hereas the trans isomer is non-polar.
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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.
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O"tical %somerism
AS Chemistry
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Learning Objectives
Candidates should be able to:
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.
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Starter activity
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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.
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Butan2!2ol
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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$#
A Light source "roduces light vibrating in all directionsB @olarising filter only allos through light vibrating in one directionC @lane "olarised light "asses through sam"le& %f substance is o"tically active it rotates the "lane "olarised light+ Analysing filter is turned so that light reaches a ma;imum
) &irection of rotation is measured coming toards the observer
A B
C DE
F
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+nantiomers G ho do they differ1
sually have the same chemical and
physical properties 6 but behavedifferently in presence of otherchiral compounds.
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+nantiomers G ho do they differ1
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'#@+S O) %SO4+$%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
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+lectro"hilicAddition to Alkenes
AS Chemistry
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Learning Objectives
Candidates should be able to:
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.
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Starter activity
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CH2=CH2 + Br2 CH2BrCH2Br
+lectro"hilic addition
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+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
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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
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+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-
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+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.
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Addition to unsymmetrical alkenes
+lectro"hilic addition to "ro"ene
2-bromopropane
1-bromopropane
Additi t t i l lk
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%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 *
Addition to unsymmetrical alkenes
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'
Addition to unsymmetrical alkenes
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@olymerisation
AS Chemistry
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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.
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Starter activity
@olyethene
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@olyethene
'em"erature: about 2//GC
@ressure:about 2/// atmospheres
%nitiator: often a small amount of oxygenas an impurity
Conditions
)ree radical addition
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)ree radical addition
#nitiation
Aropagation
"ermination
Aropagation
L&@+ or H&@+
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L&@+ or H&@+
L&@+ or H&@+
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L&@+ or H&@+
;reeFer bags, $ater pipes, $ire and cable insulation,extrusion coating
and$ich bags, cling $rap, car covers, sDueeFe bottles,liners for tan(s and ponds, moisture barriers in
construction
@olymerisation of alkenes
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@olymerisation of alkenes
+'H+(+ @OL#+'H+(+
'+'$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
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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 $hich can then be turned
&is"osal of "olymers
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O;idation ofalkenes
AS Chemistry
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Learning Objectives
Candidates should be able to describe the o;idationof alkenes by:
cold, dilute, acidified manganateB##! ions to formthe diol, and
hot, concentrated, acidified manganateB##! ionsleading to the rupture of the carbon-to-carbondouble bond in order to determine the position of
al(ene lin(ages in larger molecules.
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Starter activity
O;idation of alkenes
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O;idation of alkenes
#n the presence of diluteacidified or al(aline! potassiummanganate B##!.
&l(enes react readily at room temperature i.e. in the
cold!."he purple colour disappears and a diol is formed.
CH2=CH2 8 H2> 8 J>K H>CH2CH2>H
ethane 6 1,2-diol
O;idation of alkenes
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O;idation of alkenes
)ragment @roduct
=CH2C>2
-CH=
&ldehyde carboxylic acid
2C=
Letone
#n the presence of a hot, concentrated solution of
acidified potassium manganate B##!, any diol formed issplit into t$o fragments $hich are oxidiFed further tocarbon dioxide, a (etone or a carboxylic acid.
O;idation of alkenes
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O;idation of alkenes1. CH2=CH2
2. CH3CH=CH2
3. CH3!2C=CH2
2 products 6 bothcontain (etone
2 products 6 one contains2 (etone groups and onecontains 2 acid groups.
1 product only
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Halogenoalkanes
AS Chemistry
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Learning Objectives
Candidates should be able to recall the chemistry ofhalogenoal(anes as exemplified by the follo$ingnucleophilic substitution reactions of bromoethane+
hydrolysis
formation of nitriles
formation of primary amines by reaction $ith
ammonia.
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Starter activity
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a. CHCl3 trichloromethaneb. CH3CHClCH32-chloropropane
c. C;3CCl3 1,1,1-trichloro-2,2,2-trifluoroethane
(aming Halogenoalkanes
F
F
Cl
Cl
F Cl
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@hysical @ro"erties
a. 1-chloropropane is polar and has permanent dipole-dipole intermolecular forces that are stronger thanthe temporary dipole-induced dipole forces in non-polar butane.
b. 1-chloropropane is polar and has permanent dipole-dipole intermolecular forces that are stronger than
the temporary dipole-induced dipole forces in non-polar butane.
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(ucleo"hilic substitution
negotiate cleveralp or
cadet tart
eat given
enticed if
chenille soup
had lie
stubs tuition
electronegativepolar
attracted
negative
deficient
nucleophiles
halide
substitution
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(ucleo"hilic substitution
"his is (no$n as an S(!reaction.
Sstands for substitution,
(for nucleophilic, and
!because the initial stage ofthe reaction involves t$o species.
( l "hili b tit ti n m h ni m
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(ucleo"hilic substitution 2 mechanism
A(%4A'%O( SHO0%(3 'H+ S(! 4+CHA(%S4
&ttac( by nucleophile is to the bac( of the molecule a$ay from the negatively charged halogen atom.
$ate of reaction
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$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
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+;"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
(ucleo"hilic substitution
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hydro;ide ion ith bromoethane
ethanolCH3CH2Br + OH
- CH3CH2OH+ Br-
aDueous!
(ucleo"hilic substitution
0ater ith bromoethane
ethanolCH3CH2Br + H2O CH3CH2OH+ HBraDueous!
"his is a slo$er reaction 6 $ater is not such a good nucleophile.
$arm
$arm
(ucleo"hilic substitution mechanism
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M 2CH3
H
BrC
H
-OH
CH3
H
OHC
H Br-
hydro;ide ion ith bromoethane
(ucleo"hilic substitution mechanism
ethanol
(ucleo"hilic substitution mechanism
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ater ith bromoethane
(ucleo"hilic substitution mechanism
ethanol
+ -CH3
H
BrC
H
Br-
H
CH3
H
OHC
H
+
CH3
H
OHC
HHBr
H2O
(ucleo"hilic substitution
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propanenitrile
CH3CH2Br + CN-(ethanol) CH3CH2CN+ Br
-
cyanide ion ith bromoethane
ammonia ith bromoethane
CH3CH2Br + NH3(ethanol) CH3CH2NH22 + NH4+Br-
(ucleo"hilic substitution
aminoethane
CH3CH2Br + NH3(ethanol) CH3CH2NH2 + HBr
reflux
Heat 7
pressure
Heat 7
pressure
(ucleo"hilic substitution mechanism
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M 2CH3
H
BrC
H
CN
-
CH3
H
CNC
H Br-
cyanide ion ith bromoethane
(ucleo"hilic substitution mechanism
propanenitrile
(ucleo"hilic substitution mechanism
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ammonia ith bromoethane
(ucleo"hilic substitution mechanism
aminoethane
+ -CH3
H
BrC
H
Br-
H
CH3
H
NH2C
H
+
CH3
H
NH2C
H
NH3
H NH3+Br -
NH3
@ast "a"er -uestion
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@ast "a"er -uestion
Cl2
U.V. /!nl"#ht$thanol"%&CNre'!
Br2
U.V. /!nl"#ht
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Substitution vs.+limination
AS Chemistry
L i Obj ti
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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
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Starter activity
'y"e of @osition of +;am"le
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y" fhalogenoalkane
fhalogeno2 grou"
+ m"
primaryat end of chain+ bromoethane
secondary in middle of chain+ 2-bromopropane
tertiary attached to a carbon atom $hichcarries no H atoms+2-bromo-2-methylpropane
S( G tertiary halogenoalkanes
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S( tert ary halogenoalkanes
ucleophilic attac( at the bac( of the molecule is
hindered by bul(y CH3 groups. "ertiary carbocation isstabilised by electron donating effect of CH3groups.
S S ! 1
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S( or S(! 1
Halogenoalkane 4echanism
Arimary 2
econdary 1 and 2
"ertiary 1
+limination
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m
ou need to be a$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
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+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
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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
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AS Chemistry
@ros and Cons
Learning Objectives
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Learning ObjectivesCandidates should be able to+
interpret the different reactivities ofhalogenoal(anes e.g. C;Cs anaesthetics flameretardants plastics $ith particular reference tohydrolysis and to the relative strengths of the C-Hal
bondsexplain the uses of fluoroal(anes and
hydrofluorooal(anes in terms of their relativechemical inertness
recognise the concern about the effect ofchlorofluoroal(anes on the oFone layer.
St rt r ctivit
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Starter activity
Chlorofluorocarbons C)Cs
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Chlorofluorocarbons 2 C)Cs
.Aroperties+
on-flammable
Eo$ toxicity
nreactive
EiDuefy easily $hen compressed
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6ses
$efrigerants
@ro"ellants for aerosols
Solvents including dry2cleaning&egreasers
'he oone layer
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'he oone layer
atural oFone layer
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atural oFone layer
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$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
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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.