Category: Organic Chemistry Objective Questions

250+ TOP MCQs on Alkadienes and Answers

Organic Chemistry Multiple Choice Questions on “Alkadienes”.

1. Identify the incorrect statement regarding alkadiene.
a) These are acyclic acids
b) These are unsaturated hydrocarbons
c) These compounds have only one C=C bonds
d) These compounds have the general formula CnH2n-2
Answer: c
Clarification: Alkadiene compounds have a minimum of two double bonded carbon atoms but they can have more than two.

2. Alkadienes are classified into how many types?
a) 1
b) 2
c) 3
d) 4
Answer: c
Clarification: Based on the position and location of the double bonds, they are classified into three types.

3. Identify the incorrect statement regarding conjugated double bonds in alkadiene.
a) 1,3 alkadienes have conjugated double bond
b) Compounds with a double bond exhibit this type of bond
c) 1,3 pentadiene is an example for this bond
d) The conjugated dienes have properties similar to that of alkenes
Answer: b
Clarification: Compounds with alternative double and single bonds exhibit this kind of bond.

4. Identify the one which is the perfect example for Isolated double bond?
a) 1,4 pentadiene
b) 1,2 pentadiene
c) 1,3 pentadiene
d) 1,5 butadiene
Answer: a
Clarification: Only the 1,4 alkadiene compounds exhibit isolated double bonds.

5. Identify the incorrect statement regarding alkadienes.
a) Dienes show cis-trans isomerism
b) Conjugated dienes have better stability compared to other dienes
c) Dienophile supports alkadienes
d) Alkadienes also undergo hydrobromination
Answer: c
Clarification: Dienophiles attack the performance of alkadienes rather than supporting them.

6. Conjugated diene reacts with which among the following to form a cyclohexene?
a) Phenol
b) Dienophile
c) Hexane
d) Tribromo phenol
Answer: b
Clarification: Dienophile on reaction with conjugated dienes forms cyclohexene and this reaction is known as Diels-alder reaction.

7. Which among the following dienes undergo addition with the help of radical-chain mechanism?
a) Cumulated dienes
b) Isolated dienes
c) Simple dienes
d) Conjugated dienes
Answer: d
Clarification: Conjugated dienes undergoes addition reactions and the product usually formed are 1,4 dienes.

8. Identify the statement which is related to Diels-Alder reaction?
a) It is very stereospecific
b) Molecular distortion takes place
c) Cyclic dienes react very slow than the linear chain dienes
d) Addition of maleic anhydride to cyclopentadiene causes diene and dienophile to produce different products
Answer: c
Clarification: Cyclic dienes are more reactive than linear chain dienes.

9. A molecule in which more than one single bond separates two double bonds are called as ____________
a) Coordinate bond
b) Isolated double bond
c) Cumulative double bond
d) Conjugated double bond
Answer: b
Clarification: As per the definition of isolation, a double bond is separated by more than a single bond.

10. Isolated dienes are similar in property to ______________
a) Monoolefins
b) Diolefins
c) Triolefins
d) Tetraolefins
Answer: a
Clarification: Isolated dienes behave more like monoolefins and have properties similar to them.

250+ TOP MCQs on Organometallic Compounds – 2 and Answers

Organic Chemistry Multiple Choice Questions & Answers on “Organometallic Compounds – 2”.

1. Which of the following statement is not true about ferrocene?
a) decamethyl ferrocene is staggered in solid state
b) cyclopentadienyl rings in ferrocene are almost eclipsed
c) cyclopentadienyl ring in ferrocene are staggered
d) ferrocene can be nitrated by reaction with dil. HNO3
Answer: c
Clarification: Cyclopentadienyl ring in ferrocene are staggered is a wrong statement. Cyclopentadienyl rings in ferrocene are eclipsed.

2. Ferrocene cannot undergo which of the following reaction?
a) Friedal craft acylation
b) Diels-Alder reaction
c) Oxidation by Ag + ions
d) Electrophilic substitution
Answer: b
Clarification: Ferrocene cannot undergo Diels-Alder reaction because it does not satisfy requirements of Diels-Alder reaction (dienophile).

3. Structurally nickelocene is similar to ferrocene but how Nickelocene attains stability?
a) due to formation of a monocation
b) due to formation of a dication
c) due to formation of a monoanion
d) due to formation of a dianion
Answer: b
Clarification: Nickelocene follows 20 e– rule. S, it attains stability by formation of dication.

4. Which of the following is/are true about ferrocene?

I. Ferrocene is diamagnetic
II. Dipole moment is zero 
III.  Kealy and Pauson synthesizes ferrocene from C5H5 and freshly reduced Fe at 300℃
IV. Kealy and Pauson synthesises ferrocene from C5H5MgBr and FeCl3.

a) III and IV
b) I, II and III only
c) I, II and IV only
d) II, III and IV only
Answer: b
Clarification: Ferrocene is diamagnetic, Dipole moment is zero and Kealy and Pauson synthesizes ferrocene from C5H5 and freshly reduced at 300℃. But Kealy and Pauson does not synthesise ferrocene from C5H5MgBr and FeCl3.

5. Which type of boranes are B5H9 and B4H10 respectively?
a) Nido and Arachano boranes
b) Nido and closo boranes
c) Closo and Arachano boranes
d) Both are Nido
Answer: a
Clarification: As shown below B5H9 is Nido and B4H10 is Archano. SIngle-cluster boranes have the following general formulae, where “n” is the number of boron atoms: Nindo- BnHn+4 and Archano- BnHn+6.
B5H9 → (BH)5H4
(2(5)+4)/2=5+x
x = 2, Nido
And, B4H10 → (BH)4H6
(2(4)+6)/2=6+x
x = 3, Archano.

6. Mn(CO)5 is isolobal with which of the following compound?
a) CH4
b) CH3
c) CH2
d) CH
Answer: d
Clarification: Two fragments are isolobal if the number, symmetry properties, approximate energy, and shape of the frontier orbitals and the number of electrons in them are similar.
[Mn(CO)5]: VEC = 7 + 5 × 2 = 17 → Stable No. 18
CH3: VEC = 4 + 3 = 7 → Stable No. 8
Both have deficiency of 1 e– from their stable no. Thus, [Mn(CO)5] & CH3 are isolobal.

7. Which of following pair is not isolobal?
a) Mn(CO)5, CH3
b) [Fe(CO)4], O
c) Mn(CO)5, Cl
d) Mn(CO)5, O
Answer: d
Clarification: [Mn(CO)5], CH3 are isolobal (Deficiency of 1 e–)
[Fe(CO)4]: VEC = 8 + 4 × 2 = 16 O → 6
Deficiency of 2e– from stable no., thus they are isolobal.
[Mn(CO)5, Cl] are isolobal (Deficiency of 1 e–).
[Mn(CO)5]: VEC = 17 (Deficiency of 1)
O: VEC = 6 (Deficiency of 2)
Thus, [Mn(CO)5], O are not isolobal.

8. According to wade’s Rule, [C2B10H12] adopts which type of structure?
a) closo structure
b) nido structure
c) archano structure
d) hypo structure
Answer: a
Clarification: C2B10H12 → (BH)10H2
(2(10)+2)/2 = 10+x
x = 1, Closo structure.

9. Which property is the same for isolobal molecules?
a) e– capture
b) Boiling point
c) Melting point
d) Solubility
Answer: a
Clarification: Isolobal molecules have same deficiency of e– from their stable no. Thus, they have same e–.

10. Which of the following is not considered as an organometallic compound?
a) Ferrocene
b) Cis-platin
c) Ziese’s salt
d) Grignard reagent
Answer: b
Clarification: Cis-platin does not have metal-carbon bond. Therefore, cis platin not considered as organometallic chemistry.

250+ TOP MCQs on Reaction of Alcohols and Answers

Organic Chemistry Multiple Choice Questions on “Reaction of Alcohols”.

1. Which one of the following compound is obtained by the oxidation of primary alcohol with nascent oxygen?
a) Alkanal
b) Alkanone
c) Ether
d) Amine
Answer: a
Clarification: Oxidation of alcohols to aldehydes is partial oxidation; aldehydes are further oxidized to carboxylic acids. Conditions required for making aldehydes are heat and distillation. In aldehyde formation, the temperature of the reaction should be kept above the boiling point of the aldehyde and below the boiling point of the alcohol.

2. Which one of the following compound is obtained by the oxidation of secondary alcohols by [O]?
a) Ketone
b) Aldehyde
c) Ether
d) Amine
Answer: a
Clarification: Ketone compound is obtained by the oxidation of secondary alcohols by nascent oxygen atom [O].

3. Secondary alcohols on catalytic dehydrogenation by Cu-Ni couple gives ___________
a) Ketone
b) Aldehyde
c) Carboxylic acid
d) Amine
Answer: a
Clarification: When the vapours of a secondary alcohol are passed over copper heated at 573K, the corresponding ketone is formed.

4. The reaction of carboxylic acids with alcohols catalysed by conc. H2SO4 is called __________
a) Dehydration
b) Saponification
c) Esterification
d) Neutralization
Answer: c
Clarification: When a carboxylic acid is treated with an alcohol and an acid catalyst, an ester is formed (along with water). This reaction is called the esterification.

5. Which one is the correct order of reactivity of different types of alcohol towards hydrogen halide?
a) 1° alcohol > 2° alcohol > 3° alcohol
b) 2° alcohol > 1° alcohol > 3° alcohol
c) 3° alcohol > 1° alcohol > 2° alcohol
d) 3° alcohol > 2° alcohol > 1° alcohol
Answer: d
Clarification: Reaction of HX with alcohol depends on the degree of carbonation generated due to heterocyclic cleavage of HX bond. If alcohol is primary alcohol then reaction will proceed through Sn2 for tertiary alcohol it will proceed with Sn1. For secondary alcohol it can go through both path (Sn1 or Sn2). Sn1 being ionic in nature takes place very fast while Sn2 is molecular in nature reacts slowly. Hence, we can sum up above, Rate of reaction is 3° alcohol > 2° alcohol > 1° alcohol.

6. The dehydration of alcohols is an example of ________
a) Bimolecular elimination/E2 reaction
b) SN2 reaction
c) SN1 reaction
d) Unimolecular elimination/E1 reaction
Answer: d
Clarification: Alcohol upon reaction with protic acids tends to lose a molecule of water to form alkenes. These reactions are known as dehydration of alcohols. Dehydration of alcohols is an example of Unimolecular elimination reaction.

7. Among the following, which is least acidic?
a) Phenol
b) O-cresol
c) p-nitrophenol
d) p-chlorophenol
Answer: b
Clarification: O-cresol is least acidic because Presence of electron withdrawing groups at ortho and para position increases the acidic strength, whereas electron releasing or donating groups at ortho and para position decreases acidic strength. In cresol (ortho or para cresol), methyl group is +I group, so it destabilises the anion formed after ionisation, hence cresol is less acidic than carbolic acid (phenol).

8. Which of the following alcohols would be most soluble in water?
a) Propanol
b) Hexanol
c) Pentanol
d) Butanol
Answer: a
Clarification: Propanol is the most soluble in water among given alcohols, as solubility of alcohols in water decreases with increase in molecular mass.

9. What is the best reagent to convert isopropyl alcohol to isopropyl bromide?
organic-chemistry-questions-answers-reaction-alcohols -q9
a) HBr
b) SOBr2
c) Br2
d) CH3MgBr
Answer: b
Clarification: Thionyl bromide will react with secondary alcohols and give a reasonable percent yield of the bromide.

10. The major reason that phenol is a better Bronsted acid than cyclohexanol is ____________
a) it is a better proton donor
b) the cyclohexyl group is an electron donating group by induction, which destabilizes the anion formed in the reaction
c) phenol is able to stabilize the anion formed in the reaction by resonance
d) the phenyl group is an electron withdrawing group by induction, which stabilizes the anion formed in the reaction
Answer: c
Clarification: Whenever resonance exists, it usually is a very important factor to be considered in decisions about structure and reactivity correlations and in many cases, it is the most important factor. In this case there is delocalization of the electrons into the benzene ring.

250+ TOP MCQs on Preparation of Benzaldehyde & Aromatic Ketones and Answers

Advanced Organic Chemistry Questions and Answers on “Preparation of Benzaldehyde & Aromatic Ketones”.

1. The oxidation of toluene to benzaldehyde by chromyl chloride is called as which of the following?
a) Cannizzaro reaction
b) Wurtz reaction
c) Etard reaction
d) Reimer-Tiemann reaction
Answer: c
Clarification: The oxidation of toluene to benzaldehyde by chromyl chloride is called as Etard reaction.
advanced-organic-chemistry-questions-answers-q1

2. Benzaldehyde can be prepared by oxidation of toluene by which of the following reagent?
a) Acidic KMnO4
b) K2Cr2O7
c) CrO2Cl2
d) basic KMnO4
Answer: c
Clarification: Benzaldehyde can be prepared by oxidation of toluene by CrO2Cl2.
advanced-organic-chemistry-questions-answers-q2

3. The oxidation of benzyl chloride with lead nitrate gives which of the following compound?
a) Benzaldehyde
b) Benzyl alcohol
c) Benzoic acid
d) p-chlorobenzaldehyde
Answer: a
Clarification: The oxidation of benzyl chloride with lead nitrate gives benzaldehyde as shown in given reaction. Pb(NO3)2 is a mild oxidizing agent so benzaldehyde is formed during the oxidation of benzyl chloride with Pb(NO3)2.

4. Which of the following is the commercial method of preparation of benzaldehyde?
a) Oxidation of toluene
b) Oxidation of benzyl chloride
c) Oxidation of benzyl alcohol
d) Etard reaction
Answer: c
Clarification: This involves the treatment of benzyl alcohol with dil. HNO3 or acidic potassium dichromate or chromic anhydride in acetic anhydride or with copper catalyst at 350o C. This process is used for commercial production of benzaldehyde.

5. Which of the following is the not a method of preparation of benzaldehyde?
a) Gattermann Koch synthesis
b) Etards reaction
c) Stephan’s reaction
d) Oxidation of secondary alcohol
Answer: d
Clarification: Oxidation of secondary ketones is the method of formation of ketones not aldehydes.
advanced-organic-chemistry-questions-answers-q5

6. What reagents will be used in the preparation of benzaldehyde via Gattermann Koch synthesis?
a) Carbon dioxide and HCl
b) Carbon monoxide and HCl
c) Oxygen and H2SO4
d) Carbon monoxide and H2SO4
Answer: b
Clarification: Benzene is converted into benzaldehyde by passing a mixture of carbon monoxide and HCl gas under high pressure into the ether solution of benzene in presence of anhydrous aluminum chloride and cuprous chloride.

7. Partial reduction of phenyl cyanide with stannous chloride and passing dry HCl gas in ether solution followed by hydrolysis of the aldimine stannic chloride with water to form benzaldehyde is called as which of the following method of preparation of benzaldehyde?
a) Gattermann Koch synthesis
b) Etards reaction
c) Stephan’s reaction
d) Gattermann reaction
Answer: c
Clarification: Partial reduction of phenyl cyanide with stannous chloride and passing dry HCl gas in ether solution followed by hydrolysis of the aldimine stannic chloride with water to form benzaldehyde is called as Stephan’s reaction.

8. Ozonolysis of styrene will lead leads to formation of benzaldehyde along which compound?
a) O2
b) H2O2
c) HCHO
d) H2O2 and HCHO
Answer: c
Clarification: Ozonolysis of styrene will lead leads to formation of benzaldehyde H2O2 and HCHO.
advanced-organic-chemistry-questions-answers-q8

9. Which of the following cannot be used in formation of benzaldehyde by Grignard reagent?
a) HCN
b) Carbon monoxide
c) Ethyl format
d) HNC
Answer: c
Clarification: Reagents like carbon monoxide or HCN and ethyl format can be used for formation of benzaldehyde by Grignard reagent.
advanced-organic-chemistry-questions-answers-q9

10. What is the laboratory method for the formation of benzaldehyde?
a) Gattermann Koch synthesis
b) Etards reaction
c) Stephan’s reaction
d) Oxidation of benzyl chloride
Answer: d
Clarification: Benzaldehyde conveniently prepared by boiling benzyl chloride with copper nitrate or lead nitrate solution in a current of carbon dioxide.

11. Formation of aromatic ketone forms from benzene by CH3COCl?
a) Friedal craft alkylation
b) Friedal craft dealkylation
c) Friedal craft acylation
d) Friedal craft hydroxyalkylation
Answer: c
Clarification: Friedal craft acylation froms aromatic ketone. A proper choice of acid chloride here gives the desired ketone. The method works for all types of ketones though the bulkier aryl acid chlorides may require stronger / longer heating.

12. What will be the product for the given reaction?
a) Propanone
b) Acetophenone
c) Diphenylmethanone
d) Aliphatic and aromatic
Answer: d
Clarification: Heating solid calcium salts of benzoic acid with those of any other carboxylic acid, except methanoic acid, gives the ketones in low yields.
advanced-organic-chemistry-questions-answers-q12

13. What will be the product of the given reaction?
advanced-organic-chemistry-questions-answers-q13
a) Propanone
b) Acetophenone
c) Diphenylmethanone
d) Aliphatic and aromatic
Answer: b
Clarification: Aromatic ketones were efficiently prepared in good yields by the reactions of aryl bromides with n-BuLi, followed by the reactions with aromatic aldehydes or aliphatic aldehydes and the subsequent treatment with molecular iodine and K2CO3, in a one-pot method.

14. Which reactant can be used to form aromatic ketone for the following reaction?
advanced-organic-chemistry-questions-answers-q14
a) Arenes
b) Aldehyde
c) Alcohol
d) Terperens
Answer: a
Clarification: Aromatic ketones were efficiently prepared in good yields by the reactions of arenes with n-BuLi, followed by the reactions with aromatic aldehydes or aliphatic aldehydes and the subsequent treatment with molecular iodine and K2CO3, in a one-pot method.

15. Aromatic ketones were synthesized from aromatic compounds via liquid‐phase oxidation at 60 °C and 1 atm over vanadium‐containing which catalyst?
a) MCM-41
b) MCM-48
c) ZSM-5
d) ZK-5
Answer: a
Clarification: Aromatic ketones were synthesized from aromatic compounds via liquid‐phase oxidation at 60 °C and 1 atm over vanadium‐containing MCM‐41 catalysts using a batch reactor. The catalysts were prepared by direct hydrothermal (4V‐MCM‐41) and wet impregnation (9V/MCM‐41) methods.

16. Which of the following cannot be used as reactant in preparation of aromatic ketones from diazonium salt?
a) formaldoxime
b) Acetaldoxime
c) propionaldoxime
d) Terpernes
Answer: d
Clarification: Diazonium salts react with formaldoxime, acetaldoxime, propionaldoxime, and acetaldehyde semicarbazone severally, under carefully controlled conditions, to give arylated derivatives which can be hydrolysed to aromatic ketones.

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250+ TOP MCQs on Esters and Answers

Organic Chemistry Multiple Choice Questions on “Esters”.

1. Which of the following compounds is an ester?
a) organic-chemistry-questions-answers-esters-q1a
b) organic-chemistry-questions-answers-esters-q1b
c) organic-chemistry-questions-answers-esters-q1c
d) organic-chemistry-questions-answers-esters-q1d
Answer: c
Clarification: Esters are characterized by their possession of a modified carboxyl group, in which the H of the -OH group is replaced by an alkyl group, to form the general functional group -COOR.

2. Which of the following is used as catalyst for the esterification of carboxylic acid and alcohol?
a) Nitrous acid
b) Sulphuric acid
c) Sulphurous acid
d) Nitric acid
Answer: b
Clarification: The classic synthesis is the Fischer esterification, which involves treating a carboxylic acid with an alcohol in the presence of a dehydrating agent:
RCO2H + R′OH ⇌ RCO2R′ + H2O
The equilibrium constant for such reactions is about 5 for typical esters, e.g., ethyl acetate. The reaction is slow in the absence of a catalyst. Sulfuric acid is a typical catalyst for this reaction.

3. What will be the product for the given reaction?
CH3OH + CO →?
a) Ethyl formate
b) Methyl formate
c) Ethyl acetate
d) Methyl acetate
Answer: b
Clarification: The carbonylation of methanol yields methyl formate, which is the main commercial source of formic acid.
CH3OH + CO → CH3O2CH

4. Hydrolysis of ester leads to the formation of which of the following products in basic medium?
a) Ether and alcohol
b) Alcohol and sodium carboxylate
c) Aldehyde and alcohol
d) Sodium carboxylate
Answer: b
Clarification: Under basic conditions, hydroxide acts as a nucleophile, while an alkoxide is the leaving group.
organic-chemistry-questions-answers-esters-q4

5. What will be the product for the given reaction?
C2H4 + CH3CO2H + 1⁄2 O2 →?
a) Ethyl formate
b) Vinyl formate
c) Ethyl acetate
d) Methyl acetate
Answer: b
Clarification: In the presence of palladium-based catalysts, ethylene, acetic acid, and oxygen react to give vinyl acetate:
C2H4 + CH3CO2H + 1⁄2 O2 → C2H3O2CCH3 + H2O

6. What is the characteristic smell for ester?
a) Fruity like smell
b) Fish like smell
c) Rotten egg smell
d) Alcoholic smell
Answer: a
Clarification: Many esters have distinctive fruit-like odors, and many occur naturally in the essential oils of plants. This has also led to their commonplace use in artificial flavorings and fragrances when those odors aim to be mimicked.

7. Salts of carboxylate anions can be alkylating agent with alkyl halides to give esters. In the case that an alkyl chloride is used, an iodide salt can catalyze the reaction. What is the name reaction for the given reaction?
a) Favorskii rearrangement
b) Baeyer–Villiger oxidation
c) Pinner reaction
d) Finkelstein reaction
Answer: d
Clarification: Salts of carboxylate anions can be alkylating agent with alkyl halides to give esters. In the case that an alkyl chloride is used, an iodide salt can catalyze the reaction this reaction is known as Finkelstien reaction.

8. What will be the product of given reaction?
organic-chemistry-questions-answers-esters-q8
a) Ether and alcohol
b) Alcohol and sodium carboxylate
c) Aldehyde and alcohol
d) Carboxylic acid and alkene
Answer: d
Clarification: Ester pyrolysis in is a vacuum pyrolysis reaction converting esters containing a β-hydrogen atom into the corresponding carboxylic acid and the alkene.organic-chemistry-questions-answers-esters-q8a

9. Fries rearrangement reaction of phenol ester leads to the formation which type of product?
a) Ketone
b) Aldehyde
c) Alcohol
d) Alkene
Answer: a
Clarification: The Fries rearrangement is a rearrangement reaction of a phenolic ester to a hydroxy aryl ketone by catalysis of Lewis acids.

10. What is the order or solubility in water of the following compound?

i) ethyl methanoate
ii) ethyl butanoate
iii) ethyl ethanoate
iv) ethyl propanoate

a) i > ii > iii > iv
b) i > iii > iv > ii
c) i > ii > iv > iii
d) iii > i > iv > ii
Answer: b
Clarification: As chain length increases, the hydrocarbon portion forces itself between water molecules, breaking the relatively strong hydrogen bonds between water molecules without offering an energetic compensation; furthermore, the water molecules are forced into an ordered alignment along the chain, decreasing the entropy in the system. This makes the process thermodynamically less favorable, and so solubility decreases.

250+ TOP MCQs on Stereoisomers and Answers

Organic Chemistry Multiple Choice Questions on “Stereoisomers”.

1. What is the complete IUPAC name of the following substance?
organic-chemistry-questions-answers-stereoisomers-q1
a) (1S,3S)-1-methylcyclopentane-1,3-diol
b) (1R,3R)-1-methylcyclopentane-1,3-diol
c) (1S,3R)-1-methylcyclopentane-1,3-diol
d) (1R,3S)-1-methylcyclopentane-1,3-diol
Answer: d
Clarification: The IUPAC name will be (1R,3S)-1-methylcyclopentane-1,3-diol, where 1 position shows R and 3 position shows S.

2. How many stereoisomers are there for the following structure?
organic-chemistry-questions-answers-stereoisomers-q2
a) 1
b) 2
c) 3
d) 4
Answer: c
Clarification: In the case of 2,3-dihydroxybutanedioic acid, known as tartaric acid, the two chiral centers have the same four substituents and are equivalent. As a result, two of the four possible stereoisomers of this compound are identical due to a plane of symmetry, so there are only three stereoisomeric tartaric acids.

3. How many stereoisomers of 3-bromo-2-butanol, CH3CH(OH)CHBrCH3, exist?
a) 3
b) 1
c) 2
d) 4
Answer: d
Clarification: 3-Bromo-2-butanol has two chirality centers with different sets of four substituents. Each chirality center can have either an S or R configuration. For a compound with n chirality centers there is the possibility of a maximum of 2n stereoisomers. In this case there is more than one stereoisomer.

4. How many stereoisomers of 2,3-butanediol, CH3CH(OH)CH(OH)CH3, exist?
a) 3
b) 4
c) 1
d) 2
Answer: a
Clarification: 2,3-Butanediol has two chirality centers but the sets of four substituents on each chirality center are identical. Each chirality center can have either an S or R configuration. The (2R,3R) and (2S,3S) compounds are enantiomers of one another, but the structures drawn as (2R,3S) and (2S,3R) compounds are identical (this is the achiral meso compound), so there are only three different stereoisomers. For a compound with n chirality centers there is the possibility of a maximum of 2n stereoisomers. However, if the sets of four substituents on each chirality center are identical there will be fewer than 2n stereoisomers.

5. What is the complete IUPAC name of the following stereoisomer?
organic-chemistry-questions-answers-stereoisomers-q5
a) (1S)-2-ethyl-3,3-dimethylcyclopentane
b) (1R)-2-ethyl-3,3-dimethylcyclopentane
c) (1S)-1-ethyl-,3,3-diimethylcyclopentane
d) (1S)-3-ethyl-1,1-diimethylcyclopentane
Answer: c
Clarification: The IUPAC name will be (1S)-1-ethyl-,3,3-diimethylcyclopentane, where at 1 position it is S.

6. Which of the following structures represent the same stereoisomer?
organic-chemistry-questions-answers-stereoisomers-q6
a) Only 1 and 2
b) Only 2 and 3
c) 1, 2 and 3
d) Only 3 and 1
Answer: b
Clarification: It is important that you can rotate around carbon-carbon single bonds to make different conformations, and to rotate molecules in three-dimensions to compare their chirality chemistry (model kits are often useful). Another way to determine which of these structures are identical is to determine the configuration of each chirality center. In this case, for each chirality center the priority of the substituents is -Cl >; -CHClCH3 >; -CH3 >; -H. For example, the configurations of the chirality centers of compound 1 are both S. However, 1 and 3 are not identical.

7. Which of the following compounds can exhibit geometrical isomerism?
a) 1-Hexene
b) 2-Methyl-2-Pentene
c) 3-methyl-1-pentene
d) 2-Hexene
Answer: d
Clarification: There are several isomers of hexene, depending on the position and geometry of the double bond in the chain. One of the most common industrially useful isomers is 1-hexene an alpha-olefin.

8. How many number of stereoisomers possible for 2, 3-pentanediol?
a) 3
b) 4
c) 5
d) 6
Answer: b
Clarification: 2, 3-pentanediol contains two chiral atoms. So, the number of possible stereoisomers is 4 (22).

9. How many chiral stereoisomers can be drawn for CH3CHFCHFCH(CH3)2?
a) 4
b) 5
c) 6
d) 7
Answer: a
Clarification: CH3CHFCHFCH(CH3)2 contains 2 chiral atoms. So, the number of possible stereoisomers is 4 (22).

10. What is the total number of isomers, stereoisomers included, designated by the dichlorocyclopentane?
a) 7
b) 6
c) 4
d) 5
Answer: a
Clarification: 3 structural isomers, but if you are considering chirality/diastereomers, there are two chiral carbons, so the maximum number of stereoisomers is 22=4, from which cis-1,2-dichlorocyclopentane and cis-1,3-dichlorocyclopentane will have same structure in their individual 4 isomers as shown below, so total isomers will be 7.
organic-chemistry-questions-answers-stereoisomers-q10