aCIDITY

APEX INSTITUTE CHEMISTRY TARGET IIT JEE 2007 Hey Ketene, I couldn’t get why such a peculiar name you are having

Simple because I am having C=O (Keto) and C=C (ene)

XYZ

KETENE (CH2 =C=O)

QUESTION BANK ON

ACIDITY, BASICITY, H-BONDING & TAUTOMERISM A = Bronsted acid B = Bronsted base C = Lewis acid D = Lewis base E = Lewis acid & Lewis base both F = Bronsted base & Lewis base both G = Bronsted acid & Lewis base both A = RCO2H B = tBuO– C = AlCl3 D = Na (metal) E = Br2 (towards alkene & AlBr3) F = CH3MgX (towards H2O & carbonyls) G = CH3OH (towards RMgX & CH3Cl)

ACIDITY, BASICITY, H-BONDING & TAUTOMERISM Q.1 (a) (b) (c)

Write equations showing the Lewis acid-base reaction that takes place when Methyl alcohol reacts with BF3 Methyl chloride reacts with AlCl3. Dimethyl ether reacts with BF3.

Q.2

Which of the following are lewis acids & which are lewis bases?

⋅⋅ (a) CH 3CH 2 − N − CH 3 | CH 3 •• —

• (d) ••Br • ••

CH 3 | (b) CH 3 − C ⊕ | CH 3

(e) (CH3)3B

(c) (C6H5)3 P:

− (f) H :

Q.3 (a) (b) (c)

Which would you expect to be the stronger acid? Explain your reasoning in each instance. CH2ClCO2H or CHCl2CO2H CCl3CO2H or CHCl2CO2H CH2FCO2H or CH2FCH2CO2H

Q.4

(a) (b) (c) (d) (e) (f) (g)

Write equations for the acid base reaction that would occur when each of the following compounds or solution are mixed. In each case label the stronger acid & stronger base,& the weaker acid & weaker base. NaH is added to CH3OH NaNH2is added to CH3CH2OH Gaseous NH3 is added to ethyl lithium in hexane NH4Cl is added to NaNH2 in liq. NH3 (CH3)3CONa is added to H2O NaOH is added to (CH3)3 C-OH C2H5OH is added to a solution of HC ≡ C–Na+ in liquid NH3.

Q.5

CH3CH2MgBr + CH3C ≡ CH → A + B

Q.6

Choose the member of each of the following pairs of compunds that is likely to be the stronger base.

Q.7

(a) NH −2 or NH3

(b) OH– or H2O

(c) OH– or SH–

(d) CH3CH2O– or

O || CH − C − OH or CH3CH2O– (e) 2

(f) NH3 or NH ⊕ 4

Choose the member of each of the following pairs of compunds that is likely to be the weaker base. (a) H2O or H3O⊕ (b) H2S, HS–, S2– (c) Cl–, SH– (d) F–, OH–, NH2–, CH 3−

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(e) HF, H2O, NH3

(f) OH–, SH–, SeH–

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[2]

Q.8 (a) (b)

Label the reactants in these acid – base reactions as Lewis acids (electrophiles) or Lewis bases (nucleophiles). Use curved arrows to show the movement of electron pairs in the recations. •• —

••

••

•• —

• • CH 3 •O••• + CH 3 − Cl  → CH 3 − •O• − CH 3 + •• Cl • •• • •• • •+

CH 3 − O − CH 3 + | CH 3

•• •O− H •

| H

• •+

••

 → CH 3 − O•• + CH 3 − O − H | | CH 3 H • •—

(c)

• • O •O• || | • → H − C − H H − C − H + • NH 3  | + NH 3

(d)

CH 3 − N H 2 + CH 3 − CH 2 − •C• l••  → CH 3 − N H 2 − CH 2CH 3 + •• •C•l••

(e)

(CH3)3CCl + AlCl3 → (CH3)3C+ + –AlCl4

(f)

CH2 = CH2 + BF3 → B F3 − CH 2 − C H 2

••

+

••

—

••—

+

(g) (h)

—

+

—

+

B F3 − CH 2 − C H 2 + CH2 = CH2 → B F3 − CH 2 − CH 2 − CH 2 − C H 2

(i) Q.9

Predict the products of the following acid_base reactions. H2SO4 + CH3COO– l

Q.10 (a)

Methyllithium (CH3Li) is often used as a base in organic reactions. Predict the products of the following acid – base reaction. CH3CH2 – OH + CH3 – Li → What is the conjugate acid of CH3Li ? Would you expect CH3Li to be a strong base or a weak base ?

(b) Q.11

Which reagent in each pair listed here would be the more reactive Nu in a protic solvent ? (a) CH3NH– or CH3NH2 (b) H2O or H3O⊕ (c) CH3O– or CH3COO– (d) CH3SH or CH3OH (e) Ph3N or Ph3P (f) CH3COO– or –OH (g) H2S or HS– (h) NH3 or NH4⊕

Q.12

Each of these molecules is electrophile. Identify the electrophilic atom & draw a mechanism for reaction with a generalised nucleophile Nu–, giving the product in each case. (a)

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(b)

(c)

(d) Cl – Cl

(e)

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[3]

Q.13

Each of these molecules is nucleophile. Identify the nucleophilic atom & draw a mechanism for reaction with a generalised electrophile E+, giving the product in each case. (a) R – C ≡ C–

(b)

(d)

(e)

(c) NH2 – NH2

Q.14

Arrange the following compounds in order of increasing basicity.

(a)

CH3NH2, CH3 NH 3⊕ , CH3NH–

(b)

CH3O–, CH3NH–, CH3 CH −2

(c)

CH3CH = CH–, CH3CH2 CH −2 , CH3C≡C–

Q.15

Neither of these methods of making pentan-1,4 diol will work. Explain why not – what will happen instead?

(i)

CHO–CH2–CH2–CH2–OH MeMgBr  → CH 3 − CH − CH 2 − CH 2 − CH 2 followed by H + | | OH OH

(ii)

Br–CH2–CH2–CH2–OH Mg → MgBr–CH2–CH2–CH2–OH 3→ +

CH CHO

followed by H

Et 2O

CH 3 − CH − CH 2 − CH 2 − CH 2 | | OH OH Q.16

Suggest what species would be formed by each of the following combinations : (a) PhO – + CH3COOH + CH3COO –

(b)

(c) Pyridinium ion + trifluoroacetate ion Q.17

Say which pka belong to which functional group in case of following amino acids :

(i)

cysteine :

(ii)

glutamic acid :

Q.18

Record the following sets of compounds according to increasing pKa ( = – log Ka)

(a) (b) (c) (d) (e)

,

: 1.8, 8.3 & 10.8 : 2.19, 4.25, 9.67

, cyclohexane carboxylic acid.

1-butyne, 1-butene, butane Propanoic acid, 3-bromopropanoic acid, 2-nitropropanoic acid Phenol,o-nitrophenol, o-cresol Hexylamine, aniline, methylamine

Apex Institute

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[4]

Q.19

Explain which compound is the weaker base.

(a)

or

(b)

CH2 = CH – CH = CH – CH2– or CH2 = CH – CH2–

(c)

O O O O || || || || O − − C − C − OH or HO − C − C − OH

(d)

Q.20

or

Rank the following amines in increasing basic nature.

(a) (i)

(ii)

(iii)

(iv)

(i)

(ii)

(iii)

(iv)

(i)

(ii)

(iii)

(b)

(c)

Q.21 (a) (b)

Dimethyl furmamide (DMF) is an example of polar aprotic solvent, aprotic meaning it has no hydrogen atoms attached to highly electronegative atoms. Draw what you predict to be its most important resonance forms. DMF when used as the reaction solvent, greatly enhances the reactivity of nucleophiles. e.g. NaCN + CH3CH2Br → CH3CH2 C ≡ N + NaBr Suggest an explanation for this effects.

Q.22 (a)

Arrange the basic strength of the following compounds. OH– CH3COO– Cl– (i) (ii) (iii)

(b)

CH ≡ C– (i)

CH2 = CH– (ii)

CH3 CH2 – (iii)

(c)

CH2 = CHCH2NH2 (i)

CH3CH2CH2NH2 (ii)

CH ≡ C – CH2NH2 (iii)

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Q. Bank on Acidity, Basicity, H-Bonding & Tautomerism

[5]

(d) (i)

(ii)

(iii)

(e) (i) (f)

(ii)

Cl–

RCOO–

(i) Q.23

(iii) OH–

(ii)

(iii)

RO– (iv)

NH2– (v)

Set the following in increasing order of pkb :

,

(i) (ii)

CH3NH2 , (CH3)2 NH , (CH3)3 N , NH3

(iii)

,

(iv) Q.24

[In aqeous medium]

, Arrange the following in increasing acid strength :

(a) (i)

(ii)

(b)

CH3– OH (i)

(c)

(iii)

(ii)

O || CH 3 − C − OH (i)

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(iii)

O O || || HO − C − C − OH (ii)

O O || || HO − C − CH 2CH 2 − C − OH

(iii)

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[6]

Q.25 Set the following in increasing order of pk a : (i) Methane sulfonic acid, acetic acid & methanol. (ii)

CH3– CH2 – CH3 , CH2 = CH – CH3 , CH3 – CHO , CHO – CH2 – CHO , CH4

(iii)

CH3NO2 , (NO2)2 – CH2 , (NO2)3 – CH

(iv)

(v)

,

,

CH3COOH , NO2– CH2 – COOH , CH 3 − C − CH 2 − C − OH || || O O

(vi)

, H

(vii)

,

H

, H H

CHO – CH2 – CHO , CH 3 − C − CH 2 − C − CH 3 , Et − O − C − CH 2 − C − O − Et , || || || || O O O O

Et − O − C − CH 2 − C − CH 3 || || O O Q.26 (a) (b)

Cyanic acid (HO–C≡N) & isocyanic acid (H–N=C=O) differ in the positions of their electrons but their structure do not represent resonance structures. Explain Loss of a proton from cyanic acid yields the same anion as that obtained by loss of a proton from isocyanic acid. Explain.

Q.27

Draw a mechanism for this reaction. PhCHBrCHBrCO2H + NaHCO3 → PhCH=CH–Br + CO2

Q.28

Arrange the increasing order of acidic strength of the following compounds.

(a) (i) (b)

(iii)

CH2=CH–CH2CO2H (i)

(c)

(ii)

CH3OH (i)

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CH3CH2CO2H

(ii) CF3OH (ii)

(iii) CCl3–OH (iii)

Q. Bank on Acidity, Basicity, H-Bonding & Tautomerism

[7]

Q.29

Explain which is a stronger acid.

O O || || (b) CH 3 − C − CH 3 & CH 3 − C − CH 2CN

(a) CH3CH3 BrCH2NO2

OH

OH

(c)

or

(d) O=C–CH3

O=C–CH3

(e)

Q.30

or

or

Which of the following would you predict to be the stronger acid ?

(a)

or

(b)

CH3 – CH2 – CH2 – OH or CH3 – CH = CH – OH

(c)

CH3 – CH = CH – CH2 – OH or CH3 – CH = CH – OH

Q.31

Which is a stronger base? & Why. (a) ethylamine or aniline (c) phenoxide ion or ethoxide ion

(b) ethylamine or ethoxide ion (d) cyclohexylamine or aniline

Q.32

The Ka of phenylacetic acid is 5.2 × 10–5, and the pKa of propionic acid is 4.87.

(a) (b) (c)

Calculate the pKa of phenylacetic acid and the Ka of propionic acid. Which of these is the stronger acid? Predict whether the following equilibrium will favor the reactants or the products.

Q.33

Explain why the 2-methylpyrrolidine boils at a higher temperature than its isomers N- methylpyrrolidine.

Q.34

Although nitration usually causes elevation of B.P. yet the nitration of resorcinol to 2-nitroresorcinol decreases their B.P. then their parent compound.

Q.35

o-nitrophenol is sparingly soluble in water & has lower B.P. but p-nitrophenol is completely soluble in water & has high boiling point.

Q.36

When 30 ml of ethanol and 30 ml of water are mixed the volume of the resulting solution is less than 60 ml. Explain.

Apex Institute

Q. Bank on Acidity, Basicity, H-Bonding & Tautomerism

[8]

Q.37

Ammonium salts are much more soluble in water than are the corresponding sodium salts. Explain?

Q.38

HF has a dipole moment of 1.82 D, its boiling point is 19.34°C. Ethyl fluoride has an almost identical diploe moment & has a larger molecular weight, yet its B.P. is – 37.7°C. Explain.

Q.39

HCO2H & CH3CO2H exists as dimer. Explain.

Q.40

Cyclohexanol is more soluble in water then 1-hexanol.

Q.41

Explain why CH3OH and CH3CH2OH are reasonably good solvents for many ionic compounds compare the solvent properties of ethanol and pentanol towards ionic compound.

Q.42

1,5-Pentanediol is soluble & 1-Pentanol is slightly soluble in H2O.

Q.43

Compare the relative (i) Boiling points & (ii) Water solubilities of toluene & phenol. Explain why are most other monophenolic compounds insoluble in H2O.

Q.44

Which isomer (o, m or p) of hydroxacetophenone steam distills.

Q.45

Give structure for ethanolamine showing two different intramolecular H – bonds & discuss their relative importance.

Q.46

Which of the following system show H – bonding during tautomerism.

(a)

(b)

(c)

(d)

Q.47

Trans isomer of indigotin is stable w.r.t cis isomer. Explain.

Q.48

What is the attacking site of conjugate base of triketo form of phloroglucinol in protic & aprotic solvent.

Q.49

In each of the following pairs which is more stable :

(a)

(b)

l

(c)

(e)

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(d)

l

l

l

l

Q. Bank on Acidity, Basicity, H-Bonding & Tautomerism

[9]

Q.50

In each of the following pairs which is less stable :

(a)

(b)

l

(c)

(e) Q.51

(d)

l

l

l

l

In each of the following pairs which is more stable :

(a)

(b)

(d)

Q.52

(c)

l

(e)

l

l

In each of the following pairs which is less stable :

(a)

(c)

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l

l

(b)

(d)

l

l

Q. Bank on Acidity, Basicity, H-Bonding & Tautomerism

[10]

Q.53

In each of the following pairs which will have higher enol content : (a) CH3CHO and

(c)

(b)

and

(d)

and

(e)

Q.54

and

and

In each of the following pairs which will have less enol content : (a)

and

(b)

and

(c)

and

(d)

and

(e) CH 2 − CHO and CH 3 − CHO | Cl Q.55

In each of the following pairs which will have higher enol content : (a) CH − CHO and CH 2 − CHO 2 | | Cl NO 2 (c)

(d)

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(b)

and

and

and

Q. Bank on Acidity, Basicity, H-Bonding & Tautomerism

[11]

Q.56

In each of the following pairs which will have less enol content :

(a)

Q.57

(c)

and

(e)

and

Q.59

(d)

and

(e)

(a)

(b)

(d)

(e)

(c)

Which of the following compounds can exhibit tautoimerism :

(b)

(c)

(d)

(e)

Which of the following compounds can not exhibit tautoimerism :

(a)

(d) Q.61

and

Which of the following compounds can not exhibit tautoimerism :

(a) Q.60

(b)

Which of the following compounds can exhibit tautoimerism : (a) CH2 = C = O (b) CH2 = CH – OH (c) HO – CH = CH – OH (d) CH3CN

Q.58

and

(b)

(c)

(e) CH3 – NO

What is the relationship between these two molecules? Discuss the structure of the anion that would be formed by the deprotonation of each compound.

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Q. Bank on Acidity, Basicity, H-Bonding & Tautomerism

[12]

Q.62

5,5 – dimethylcyclohexane –1,3 –dione (dimedone) shows tautomerism.Write down tautomeric structure.

Q.63

Draw enol forms of these carbonyl compounds and comment on the stability of the enol forms.

(a)

Q.64

(b)

The proportion of enol in a sample of the two ketones below are shown. Why are they so different ? 4 × 10–4 % enol

(a)

Q.65

(b)

62 % enol

(a) Eugenol and (b) Saffrol Shows tautomerism, what are their tautomeric form.

Q.66

Isatin was the first compound to show tautomerism.

What is the tautomeric form of isatin. Q.67

1, 3 – dicarbonyl compounds such as (A) are usually mostly enolized. Why is this ? Draw the enols available to compounds A & B comment on the different pattern of enolization.

(A) Q.68

(B)

In each of the following sets of compounds write the increasing order of % enol content

(a)

(b)

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[13]

(c)

(d)

(e)

Q.69

In each of the following sets of compounds write the decreasing order of % enol content.

(a)

(b)

(c)

(d)

(e)

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[14]

Q.70

Q.71

Enol form of cyclobutanone is very unstable vs enol form of triketocyclobutane, which is very stable.

has lower boiling point than

even when former has – OH group.

Q.72 Among these give ease of enolization. Q.73

% enol content of acetylacetone in following solvents is found as : Solvent % enol content H2 O 15 Liquid state 76 hexane 92 gas phase 92 Explain the observation.

Q.74

Explain the observation.

Q.75

l This tautomeric system exist almost exclusively in favour of phenol and it is insensitive to change in solvent.

Q76.

CH3–CH=CH–

&& H +HBr → Pdt –O ⋅⋅

Write the product & expalin the mechanism of the reaction.

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Q. Bank on Acidity, Basicity, H-Bonding & Tautomerism

[15]

For Problem no 77 & 78 The strongest secondary force is the hydrogen bond. Hydrogen bond is of two types: Intramolecular and intermolecular hydrogen bond has great effect on melting point, boiling point & solubility. Q77. Taking into consideration the presence or absence of intermolecular hydrogen bonding which one of the following represent the correct decending order of boiling point of the following compounds

(II)

(I )

(A) III > II > I Q78.

(B) II > I > III

(III) (C) I > II > III

(D*) I > III > II

Consider the following statements about intermolecular and intramolecular hydrogen bonds (i) Both types of hydrogen bonds are temperature dependent. (ii) Intramolecular H–bond disappears on decreasing having intramolecular. (iii) The boiling point of compounds having intramolecular H–bond are lower than that of those compound & which have intermolecular hydrogen bond. Which of the statements given above are correct . (A) (i) , (ii) & (iii) (B) (iii) & (iv) (C) (i), (iii) & (iv) (D) (i) & (ii)

For Problem no 79 & 80 Q79. Acid strength is measured by the position of equilibrium of ionisation in water. In other words acid strength is function of the stability of conjugate base of the acid. More is the stability of conjugate base of the acid more is the acidity of the acid. Which of the following compound is more acidic in nature (A) Q80.

(B*)

(C)

(D) CH2 = CH–CH3

Which one of the following phenols will show highest acidity?

(A)

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(B)

(C*)

(D)

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[16]

ANSWER KEY Q.2 Q.3 Q.5 Q.6 Q.7 Q.11 Q.14 Q.18 Q.19 Q.20 Q.22 Q.23 Q.24 Q.25 Q.28 Q.29 Q.30 Q.31 Q.32 Q.49 Q.50 Q.51 Q.52 Q.53 Q.54 Q.55 Q.56 Q.57 Q.58 Q.59 Q.60 Q.68 Q.69 Q.72

LA b,e LB acdf (a) 2; (b) 1; (c) 1 CH3CH3 + CH3C ≡ CMgBr (a) 1; (b) 1; (c) 1; (d) 1; (e) 2; (f) 1 (a) 2; (b) 1; (c) 1; (d) 1; (e) 1; (f) 3 (a) 1; (b) 1; (c) 1; (d) 1; (e) 2; (f) 2; (g) 2; (h) 1 (a) 22>1>3; (c) 4>3>1>2; (d) 1>3>4>2; (e) 3>2>4>1 3>1>2 Q77. D Q78. Q79. B Q80. C