carboxylic acid questions-1 (1)

8/12/2019 Carboxylic Acid Questions-1 (1)

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Carboxylic Acid Questions

1.-Arrange the following in order of (i) increasing acidity, (ii) increasing pK a

(a) C! "C# $! (b) CCl "C# $!

(c) C! "C! $#!(d) C! "C! $%!

$.-&hat would be the 'a or products of the reactions of (i) butanoic acid and (ii) ben oic acid with each of the following

(a) %#Cl $, *t " + (b) iAl! /!0 then acidic wor -up

(c) (C! ")$C!#! ! 2 heat(d) +a#!

".-!ow could you use 1-bro'obutane to prepare each of the following carboxylic acids 3

(a) propanoic acid (b) butanoic acid(c) pentanoic acid (d) hexanoic acid

Carboxylic Acid Answers

1 (i) increasing acidity

C! "C! $#! 4 C! "C! $%! 4 C! "C# $! 4 CCl "C# $!5esonance stabili ation in carboxylates 'a es carboxylic acids 'ore acidic thanalcohols or thiols.*lectron withdrawing groups, here -Cl, increase the acidity due to furtherstabili ation of the carboxylate./hiols are 'ore acidic than alcohols due to wea er 6-! bond and the ability of %to acco''odate extra electrons (si e).(ii) increasing pKa

CCl "C# $! 4 C! "C# $! 4 C! "C! $%! 4 C! "C! $#!5e'e'ber the lower the pKa the stronger the acid, so once you ha7e part (i), thisis ust the opposite.Tip &hen as ed about pKa trends, it 'ay be easier to thin in ter's of aciditythen re'e'ber to flip the order.

$ +ote the identical nature of the reactions that the aliphatic and aro'atic acidundergo


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(a) /hionyl chloride, %#Cl $, is used to prepare acyl chlorides, the base re'o7esthe !Cl by-product.(b) iAl! is a hydride reducing agent, acids to pri'ary alcohols.(c) An alcohol and a carboxylic acid gi7e an ester.(d) Carboxylic acids react with bases to gi7e carboxylates.

" 0irst, note that we ha7e an ho'ologous series of C" to C8 acids we are trying to'a e.!ere is a sche'e collecting possible syntheses together (based on the 'orei'portant reactions)

(a) Propanoic acid need to lose a C fro' C . &e can do this 7ia o onolysis of anal ene, which we can obtain by eli'inating the al yl halide.(b) Butanoic acid oxidation of the corresponding alcohol will gi7e the carboxylicacid, so prepare the alcohol by substitution.


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(c) Pentanoic acid need to gain a C to get C9. #ne way to do this is 7ia thereaction of the :rignard reagent with carbon dioxide. Alternati7ely, substitutionwith +aC+ then hydrolysis would also wor .(d) Hexanoic acid need to gain $C to get C8. 5eaction of the :rignard reagentwith ethylene oxide gets the right nu'ber of C and a pri'ary alcohol ready for

oxidation to the acid.

More questios..

4: Arrange the following in order of decreasing reactivity towards hydrolysis


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using aqueous NaOH.

(a) CH 3CO CH 3 (!) CH 3COCl(c) CH 3CON(CH 3) (d) CH 3CO COCH 3

": #hat are the $roducts of the hydrolysis reactions considered in %u & aftera dilute acid wor' u$

*: (a) Arrange the following in order of decreasing reactivity towards +iAlH 4 ,-H followed !y dilute acid wor' u$.

(!) #hat are the $roducts of each of the reactions in $art (a) /: #hat is the $roduct of each of the following reactions

Answers:

4: Hydrolysis is a nucleo$hilic acyl su!stitution reaction0 ty$ical of car!o1ylic acidderivatives.


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irst tas' should !e to identify the functional grou$s in each 2olecule thenuse the reactivity order. t can !e rationalised !ased on (i) the interaction ofthe substituent and the car!onyl grou$0 and0 (ii) the a!ility of the substituentto function as a leaving grou$.

CH 3CO Cl CH 3CO OCOCH 3 CH 3CO OCH 3 CH 3CO N(CH 3)

acidchloride anhydride ester a2ide

": All the car!o1ylic acids in %u & are derivatives of ethanoic acid0 so they allgive the sa2e car!o1ylic acid...

(a) CH 3CO H (!) 2oles CH 3CO H(c) CH 3CO H and CH 3OH (d) CH 3CO H and HN(CH 3)

*: +iAlH4 is a source of H (a nucleo$hile) which functions as a reducing agent.irst tas' should !e to identify the functional grou$s: car!o1ylic acid0 'etone0

aldehyde0 ester.

-he aldehyde and 'etone will undergo nucleo$hilic addition0 the acid and theester nucleo$hilic acyl su!stitution.Consider the electro$hilicity of the car!onyl grou$ in each co2$ound in each$air.

Aldehydes are 2ore reactive than 'etones 555 as they are less hindered andthe al'yl grou$ in the 'etone is a wea' electron donor.

6nder the reaction condition s the car!o1ylic acid will de$rotonate to give thecar!o1ylate which is a very $oor electro$hile (after all0 it has a negativecharge 5) so the ester is 2ore reactive than the acid.Now co2!ine the two $airs. 7ince the O8 grou$ is a stronger electron donor(resonance) than the al'yl grou$ of the 'etone0 the ester is less reactive thanthe 'etone... so we get :

(!) -he aldehyde0 car!o1ylic acid and ester will !e reduced to the sa2e


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$roduct0 !en9yl alcohol. -he 'etone will !e reduced to & $henylethanol0C*H"CH(OH)CH3

/: -he answers the these questions involve 2aterials fro2 this cha$ter andreview fro2 cha$ters & and &;

carboxylic acid questions-1 (1)

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