alkenes markscheme

1.

(i)

but-1-ene (1)

(ii) (ii)

two H on on one one carb carbon on of doub double le bond bond (1)

(iii)

CH3CH=CHCH3 (1)

(iv)

C H C H

3

C

C H

2

(1)

3

[4]

2.

(a)

(b)

 Name of alkene  alkene 

3-methylpent-2-ene (1)

Type Type of stereoisomerism

geometrical or cis-trans (1)

 Name of mechanism

electrophilic addition (1)

 Mechanism

C H

C H

3

3

               "

C H

3

C H

C

C H C H

(1 )

H

2

C H

3

C H

2

C H

(1 )

3

(1 )

C 2

C H

3

!  r  #

 r (1 )

(c)

Structure

C H C H

3

3

(1 )

C H  r C H C H 2C H

 Explanation

3

(CH3CH2)2

+

C

CH3 or tertiary carbonium ion more

stable (1) than CH3 + HCH(CH3) CH2 CH3 or secondary C

carbonium ion (1) [10]

3.

 

(i (i)

C1$H3%&2 &nly' CH1& &nly' (empirical formula is not consequential on molecular formula)

(ii) (ii)

(*n unsatu unsaturat rated ed comp compoun ound) d) conta contains ins (at (at least least)) one doub double le bond bond OR Contains C=C' (must be a positive statement)

(iii (iii))

+1! +1! ro rom mine ine wate water  r  &,  r 2(a) &,  romine &,  r 2' (penalise bromi!e "ater#$ but mark on)  Ml% decolourised or goes colourless OR from brown.red.orange.yellow to colourless' (Must be colourless# not clear# for M&) (chemical error if no rea'ent or "ron' rea'ent$ loses both marks) (cre!it MnO % for M$ (purple) to colourless for M& (if aci!ifie!) OR (purple) to bro"n*bro"n precipitate (if alkaline or unspecifie!) (No cre!it for hy!ro'en or io!ine as rea'ents) [5]

4.

(a)

(i)

/lectrophilic ad addition (+oth "or!s require!) require!)

(ii) (ii)

+1 the the rea react ction ion to to form form 1-b 1-bro romo mopr prop opan anee goes goes via the via the primary o

carbocation &, 1  carbocation &, via

+

CH 3 CH 2 C H 2

+2 primary carbocations are less stable than secondary carbocations0 (,re!it converse ar'uments for M an! M& i-ethe reaction to for m &.bromopropane &.bromopropane 'oes via vi a the secon!ary carbocation $ M$ an! secon!ary carbocations are more stable than primary carbocations$ M& ) (/ccept the use of carbonium ions# as an alternative to carbocation)

(b)

(i)

econdary &, 2

o

(ii) C H

3

C H

C H H

C H

2

"

3

C H C H

+ 3 s t r u c t u r e o f c a r b o c a tio n

3

 #

&  & 2& H

!&  &

2

& H

+1 arrow from double bond to H of H # & bond +2 arrow from bond to oygen atom to show H # & bond breaage +% arrow from lone pair of electrons to carbon atom of carbocation (0enalise M if arro" 'oes to 1 &SO2 or to  formal positive char'e on 1$ but but i'nore partial char'es on sulphuric aci! unless "ron') 3

(,re!it M& for 1   ion) (4or M2$ accept ne'ative char'e any"here on the ion) (iii (iii))

Cata Cataly lyst st &45 &456 (5'nore homo'eneous$ hetero'eneous) [9]

5.

(a)

(i) (i)

the the 7oin 7oinin ing g tog togethe etherr of monom onomer erss . smal smalll molec olecul ules es (1) to form long chains . large molecules (1)

(ii)

nCH2 = CH2

→ 

(-CH2#CH2-)n (1)

allow n CH 2 CH2 not n C2H% (b)

182-dibromoethane (1)

(c)

ele electro ctroph phil ilic ic add additio ition n (1) H

H C

C

H  r  r

H

δ

"

δ

 #

H

H

H

C

C

 r

!  r  #

H

H

H

H

C

C

 r

 r

H

words or diagrams to show attac by p electrons on r atom  # 

and either δ".δ # on r 2 or e  shift on r#r (1)

correct carbocation intermediate (allow triangular representation) (1)  # 

attac by r   (onto "ve carbon) leading to correct product (1)

(d)

(i) (i)

C 3$0 3$01 1.12 .12 = 3023 3023 '

H 09$ 09$.1 = 09 09$ '

& :1091 1091.1 .19 9 = 3023 3023 (1)

ratio C!H!& C!H!& = 1!3!1 .empirical formula = CH CH 3& (1) empirical mass = 31 so molecular formula formula = 2 ; CH CH 3& = C2H9&2 (1) (ii)

reagent = 4a&H . <&H (1) conditions = aueous solution (dependent on first mar) (1)

(iii)

CH2rCH 2r " 24a&H →

CH2(&H)CH2&H " 24ar 

 product = CH2(&H)CH2&H (condone missing bracets) (1) correctly balanced (1) if C2H9&2 given8 allow second mar only for CH2 r CH 2  r " 2H 2& → CH2(&H)CH2(&H) " 2Hr allow 2 mars if reagent in (ii) is H 2& or aueous solution [15]

2.9 ALKENES EXTRA QUESTINS !S

1.

(a)

C2H9 → C2H% " H2

(b)

*ddition (i'nore self or chain as a preface to a!!ition ) (penalise a!!itional) [2]

2.

/lectrophilic addition +1! curly arrow from C=C bond towards.alongside the side of H atom on H1&&2&H (penalise M if arro" to 1 &SO2 OR to formal char'e on 1 of 1 1O bon!) (i'nore partial char'es on 1 an! O of 1 &SO2 $ but penalise if these are incorrect on the 1 atom bein' attacke!) (cre!it M an! M& if correct curly arro" to 13 provi!e! the anion is  present) +2! curly arrow from H#& bond towards.alongside the side of the & at om on H#&&2&H (cre!it the arro" even if there are partial or formal char'es on 1 an! O but the structure of 1 &SO2 is correct) +3! correct structure of the carbocation (penalise use of 6sticks7 in this structure) +%! curly arrow from lone pair on an individual oygen atom of  (correct formula for) hydrogensulphate ion towards.alongside C atom  bearing the positive charge (insist that the an ion has the correct formula "ith a lone pair of  electrons an! a ne'ative char'e)  [5]

3.

Type Type of isomerism  Explanation

geometrical or  restricted rotation or  double bond rigid (1) [2]

4.

H

3

C

C H C

C

H

H

3

3

H C

(1 )

H

C C

H

C H

(1) 3

H accept

C H

3

C

C

H

H

C H

3

and

C H

3

C

C

C H

3

H

Credit 1 mark for a correct formula for but-2-ene Credit 1 mark for any pair of cis / trans isomers

eometric(al) &r cis-trans &r diastereoisomerism N! stereoisomerism ["]

(i)

5.

" H

2

or C9H1> " H2 → C9H12

(ii)

 Rea'ent(s)

r 2 or <+n&% (1)

Observation(s)

no charge (1)  ["]

".

(i) (i)

/lec /lectr tron on pai pair. r. lon lonee pair pair acc accep epto torr &, seei seeing ng.b .bon onds ds with an electron pair  (insist on reference to a pair of electrons)

(ii) (ii)

+1 curly curly arrow arrow from from midd middle le of C=C bond bond of the the alen alenee toward towards. s. alongside the H atom of the H-r' (penalise arro"s "hich 'o to"ar!s one of the carbon atoms) (i'nore a partial ne'ative char'e on the ,8,) +2 curly arrow from H-r bond to side of r atom' (penalise M& if there are formal char'es on 1+r  or if there are partial char'es "hich are the "ron') (penalise M& if the sin'le bon! has t"o !ots in a!!ition to the line) +3 correct structure for carbocation' (penalise M9 if the positive char'e is place! on the en! of a bon!) (penalise M9 if any alkene other than ethene is use! . all other marks can  score) +% curly arrow from lone pair on bromide ion to the positive carbon of carbocation8 ensuring that bromide ion has a negative charge' [5]

#.

(a)

+l cred credit it a corr correct ect structu structure re for for eith either er geom geometr etrica icall isome isomerr and its its

designation as either cis or trans&, credit two correct geometrical isomer structures (ignore the names) &, credit two correct names for cis pent-2-ene cis pent-2-ene and trans pent-2-ene trans pent-2-ene (ignore the structures) +2 credit a second mar if all four parts of the reuired structures and names are correct0 (cre!it linear# structures) (insist on the alkyl 'roups bein' attache! clearly by ,., bon!s) (b)

(i) (i)

+l curl curly y arro arrow w fro from m mid midd dle of C = C bon bond d to to H atom atom on H- H-r  r  (penalise M if partial ne'ative char'e or  formal positive char'e on 1) (penalise Ml if pent.&.ene is use!) +2 curly arrow from H-r bond to side of r atom +3 correct structure for correct secondary s econdary carbocation +% curly arrow from lone pair on bromide ion to the positive carbon of  carbocation8 ensuring that bromide ion has a negative charge0 ("ith the exception of pent.&.ene$ if the "ron' alkene is use!$ only penalise the structure M9) (penalise the use of t"o !ots in a!!ition to a covalent bon!$ once only)

(ii)

1-bromopentan tane

(iii (iii))

+l 2-b 2-bro romo mope pent ntan anee is form formed ed via the secondary (or 2?) carbocation &, 1-bromopentane is formed via the primary (or 1?) carbocation +2 a secondary carbocation is more stable than a primary carbocation award this mar only if the uality of language 7ustifies the award0 (the ar'ument must involve clear statements about carbocations) [9]

$. (1 ) + % arrow

H (i)

3

C

H C

H (1 ) arrow + 1

! r "

C

H

H

3

C

C H

H

 #

 r

C H

3

+ 3 (1 ) c a r b o c a t i o n

( 1 )  a r r o w + 2 #f $ron% carbocation& lose structure mark #f $ron% alkene& lose structure mark Can still score ' i(e( penalise )" *en *enalis lise )2 if polar larity ity inc includ luded inco incorrrec rectly no bond bet$een + and ,r  bond is so$n as or

(ii)

"

C H 3C H 2C H

2

(1 )

credit secondary carbocation ere if primary carbocation as been used in .i             !

#%nore attack on tis carbocation by  r

 #

 [5]

9.

(a)

melti elting ng point int inc incre reaases ses (1)  boiling point increases(1) or

they are liuids8 the higher members are solids (1)

density increases(1) viscosity increases(1) (b)

addition (1)  polymerisation (1)

(c)

(i)

C2H% " H2&→ C2H:&H - must show the functional group (1)

(ii) (ii)

vapo vapour ur pha phase se . hig high h tempe tempera ratu ture re (3> (3>> > @ :>?C :>?C)) (1) high pressure >cl @ 2> (1) if high T  and  and high p high p88 then only 1 mar8 value for either gives 2nd mar  strong acidic catalyst .H 3A&% (1)

(iii (iii))

elec electr trop ophi hili licc (1) addition (1) [10]

(i)

1%.

 5 s o m e r )

C l

C l C

C l (1 )

C

H

Bc re d it

5so m e r &

e i th e r o r d e r

C

H

H

H

H C l

C l

C

C

(1 )

C C l C l

H

and

H

C

C

H 

C l

(ii) (ii)

restri restricte cted d rota rotation tion &, no rotati rotation on &, canno cannott rotate rotate (1)  ["]

11.

(a)

e l e c t r o p h i l ic

a d d i t i o n (1)

H

2 C

(1 )

C H

H

2

C l C l

(1 )

(1 )

" C 2

            !

C l

 #

C H C l

(1 ) 2

C H (b)

C 2H

%

C l

2

or

2

C l

C H

H

2

H C

C l

H

C

"

H C l

C l

(1)

(c)

este esterr or alo loy y alco alcoho holl (1)

(d)

(i)

H&#CH2 #CH2 #&H (1)

 (ii)  (ii)

high high elec electro tron n dens density ity of double double bond bond (1)  # 

repels &H  or nucleophile (1) [10]

12.

(a) (a)

geom geomet etri rica call or ciscis-tr tran anss isom isomer erss (1) due to restricted rotation (1)

(b) (i)

C H  rC H

2

C H

3

(1 )

C H

2

C H  rC H

3

(1 )

(ii) (ii)

elec electr trop ophi hili licc addi additi tion on (1)

(iii)

C9H:

+

C

H CH2CH3 (1)

+

C9H:CH2 C H CH3 (1)

 both secondary but one is more stable (1) []

13.

(i) (i)

the the sam samee gen gener eral al form formu ula or C nH2n . the same functional group . a C=C . a double bond . differ from their immediate neighbour by CH 2 allo" 6all strai'ht chain alkenes7 

(ii)

increases not 7ust Dpent-l-ene highestE' allow Dethene lowest8 pent-l-ene highestE

(iii (iii))

methy methylpr lprop open enee . 2#meth 2#methyl ylpr prop open enee not 2-methylprop-2-ene (1) i%nore $ron% punctuation

C H

H C

C

3

H C

H C

H

or

H

C H

3

C H

3

C

H

H allo"  mark for but.&.ene "ith its correct structure (1)

m i n i m u m s t r u c tu r e i s C H

2

= C C H C H

3

or C H

2

= C (C H

3

)C H

3

3

[4]

14.

,atalyst (c) phosphoric acid or (c) sulphuric acid (1)  4ot dilute accept correct formula ,on!itions

Femp = High or 2>>#:>>?C (1)

!emp  medium or moderate or 50-100C

Aressure = High or :#2> +pa or :>#2>> atoms *ressure  +i% or 234 )pa or 20340 atoms

 5f- "ron'$ no catalyst catalyst 'iven$ allo" phosphoric aci! con!itions

 Equation

CH2 = CH2 " H2& → CH3 CH2 &H

allo" , & 1 2

 allo" , & 1 :O1 

not ,1 & ,1 & [4]

15.

+1 G is 182-dibromoethane only +2 electrophilic addition (both "or!s nee!e!) +3 the double bond is a centre of electron density &, electron-rich &, nucleophilic &, a source of an electron pair  &, a pi cloud.bond of electrons +% a dipole or polarity is induced.created.formed in the r-r  bond.molecule award this mar only if the uality of language 7ustifies the award0  [15]

*n appropriate alene' CH 3CH2CHCH 2 or (CH 3)2CCH2 1 somer 1

1".

1 somer 2 1 Aosition isomerism 1 +echanism "

electrophilic attac and electron shift to r (Inless H  used) carbocation reaction with carbocation ;/llo" mechanism marks for the alkene ,1 3,1,1,1 3 < ;/llo" one mark if mechanism for minor  pro!uct 'iven< []

1#.

(a)

,eaction 1

H2& or steam (1)

(b)

H

2

C (1 )

CH

+ 1

H

H

2

 r (1 )

(1 )

H

H

C "

C

H

(1 )

+ % s tru c tu re

H ! r

 #

+ 2 + 3  0enalise M& incorrect δ 3 * δ  =   0enalise δ  = on alkene (M)  0enalise !ots on bon!s once  0enalise M2 (structure) for use of "ron' "ron' alkene  0enalise M for use of +r &  [5]

1$.

etha ethane ne

tetr tetrah ahed edra rall (or (or 3d show shown n in in dia diagr gram am)) (1)

1>( 12 )?

(1)

ethene

(trigonal) planar (1)

12>?

(1)

 bond lengths! C # C in ethane longest or C = C in ethene shortest

J J (1) density

in ethe ethene ne !

2 pairs pairs of e.grea e.greater ter electr electron on

.π bond (1)  []

19.

(a) (1 )

H

(1 )

(C H

3

)2 C

 r

C H C H

2

C H

"

(C H

3

3

)2C

C H

(1 )

!  r  #

2

C H

2

C H

3

(C H

(1)

3

)2C

C H

2

C H

2

C H

3

(1 )

 r

2#bromo#2#methylpentane (1) tertiary carbonium ion more stable than secondary (1)

(b)

geome geometric trical al or cis#tra cis#trans ns isome isomerisa risation tion (1) he#3#ene or he#2#ene (1) ,

1

, C

2

, (1 )

C

H

H C

H

(c is )

1

H

(1 )

C ,

(tra n s)

2

 [11]

2%. (1 )

C H

C H (1 ) 3

2

H

 r

C H

= C

(1 )

C H

C H

3

C H

3

3

C H 2C H (1 )

2

!  r  #

(1 ) " C

C H

C H

3

 r C H

3

3

C H 2C H (1 )

2

C C H

electrophilic addn (1) tertiary (1) 2-bromo-2-methylpentane (1) or

" C H 3 C H 2C H # C H C H (1 )

C H

secondary (1)

3

3

C H 3 C H 2C H # C H C H (1 )

 r

C H

3

3

3-bromo-2-methylpentane (1)

tertiary more stable than secondary (1)  [1"]

21.

(a)

A & alene (1) ' & halogenoalane . bromoalane . alyl halide . haloalane (1)

secondary) (1)  # alcohol (ignore primary8 secondary) (b) (b)

(c) (c)

(i) (i)

addi additi tion on igno ignore re nucl nucleo eoph phil ilic ic . elec electr trop ophi hili licc . free free radi radica call (1)

(ii)

substitution not replacement . displacement (1)

odi odium um hydr hydro oid idee . 4a&H 4a&H . <&H <&H not 7ust hydroide (1) (' to ) (' to A)

aueous not dilute (1) alcoholic (1)

mar alternatives as (d) ignore references to concentration and temperature

C H 3

3

(d)

(i)

CH3CH(CH3)r " 4a&H → CH3CH=CH2 " 4ar " H 2& (1)

(ii)

CH3CH(CH3)r " 4a&H → CH3CH(CH3)&H " 4ar  (1) allow molecular formulae C 3Hr' C3H$&' C3H9  # 

 # 

allow ionic versions (with &H 8 r  ) (e)

 # 

arrow from & of &H  to C 7oined to r  (1) lone pair not needed C#r polarity shown by δ" δ # or heterolytic fission of C#r bond shown by arrow from bond between C and r to r or intermediate with partial bonds and minus sign (1)  # 

r   as product (1) allow all 3 mars if 1-bromopropane identified as ' [1"]