Most important 25 short questions with answers that are frequently encountered by graduation students in organic chemistry:
1. What is meant by inductive effect? Give an example.
Answer: The inductive effect is the permanent displacement of sigma (σ) electrons along a saturated carbon chain due to the difference in electronegativity between atoms. For example, in ethyl chloride (CH3CH2Cl), the chlorine atom is more electronegative than carbon, so it pulls the σ electrons towards itself, creating a partial positive charge (δ+) on the adjacent carbon atoms and a partial negative charge (δ−) on the chlorine atom. This polarization is the inductive effect.
2. Define hyperconjugation.
Answer: Hyperconjugation is the delocalization of σ electrons of a C-H bond of an alkyl group directly attached to an unsaturated system (like a carbocation, radical, or alkene) or an atom with an unshared p-orbital. This overlap of a σ bonding orbital with a neighboring empty or partially filled p-orbital or a π bonding orbital stabilizes the system. It's also known as "no-bond resonance."
3. What are enantiomers? Give an example.
Answer: Enantiomers are stereoisomers that are non-superimposable mirror images of each other. They have the same molecular formula and connectivity of atoms but differ in their spatial arrangement. For example, (+)-lactic acid and (-)-lactic acid are enantiomers.
4. Explain SN1 reaction mechanism.
Answer: The SN1 (Substitution Nucleophilic Unimolecular) reaction proceeds in two steps:
Step 1 (Slow): Heterolytic cleavage of the C-Leaving group bond, generating a carbocation intermediate. This step is unimolecular as only the substrate is involved in the rate-determining step.
Step 2 (Fast): Attack of the nucleophile on the carbocation, forming the substituted product. The carbocation is planar, leading to racemization if the carbon center is chiral.
5. What is Markovnikov's rule? Explain with an example.
Answer: Markovnikov's rule states that in the addition of a protic acid (HX) to an unsymmetrical alkene, the hydrogen atom of HX adds to the carbon atom of the double bond that already has a greater number of hydrogen atoms, and the X group adds to the carbon atom with fewer hydrogen atoms.
Example: The addition of HBr to propene (CH3CH=CH2) yields 2-bromopropane (CH3CHBrCH3) as the major product, where the hydrogen adds to the CH2 group (more hydrogens) and bromine adds to the CH group (fewer hydrogens).
6. What are Grignard reagents? How are they prepared? Give one important use.
Answer: Grignard reagents are organomagnesium halides with the general formula RMgX, where R is an alkyl or aryl group and X is a halogen (Cl, Br, or I).
Preparation: They are prepared by the reaction of an alkyl or aryl halide with magnesium metal in dry ether or tetrahydrofuran (THF) as a solvent: R−X+Mgdryether/THFR−Mg−X
Important Use: Grignard reagents are powerful nucleophiles and strong bases, widely used in organic synthesis for the formation of carbon-carbon bonds by reacting with carbonyl compounds (aldehydes, ketones, esters, carbon dioxide), epoxides, and other electrophiles to yield alcohols, carboxylic acids, etc.
7. Explain the acidity of phenols.
Answer: Phenols are more acidic than alcohols due to the stabilization of the phenoxide ion (the conjugate base) through resonance. When a phenol loses a proton, the negative charge on the oxygen atom is delocalized into the benzene ring through resonance, making the phenoxide ion more stable and thus the phenol more acidic. Alcohols do not have such resonance stabilization in their conjugate bases (alkoxides), making them less acidic.
8. What is aldol condensation? Give the general reaction.
Answer: Aldol condensation is a reaction in which an enolate ion reacts with a carbonyl compound (aldehyde or ketone) to form a β-hydroxy aldehyde or β-hydroxy ketone, followed by dehydration to give an α,β-unsaturated carbonyl compound. The reaction is typically catalyzed by a base or an acid.
General Reaction: 2RCH2CHOBaseorAcidRCH2CH(OH)CH(R)CHO−H2ORCH=CH−CHO (Aldehyde example)
9. What is the Hofmann bromamide degradation reaction?
Answer: The Hofmann bromamide degradation reaction is a method for converting a primary amide to a primary amine with one less carbon atom. The amide is treated with bromine in an aqueous or alcoholic solution of sodium hydroxide. The reaction proceeds via an isocyanate intermediate.
General Reaction: RCONH2+Br2+4NaOH⟶RNH2+Na2CO3+2NaBr+2H2O
10. Differentiate between SN1 and SN2 reactions.
Answer:
|
Feature |
SN1 (Unimolecular Nucleophilic Substitution) |
SN2 (Bimolecular Nucleophilic Substitution) |
|
Mechanism |
Two steps (carbocation intermediate) |
One step (concerted) |
|
Rate Law |
Rate = k[Substrate] |
Rate = k[Substrate][Nucleophile] |
|
Molecularity |
Unimolecular |
Bimolecular |
|
Stereochemistry |
Racemization (if chiral center) |
Inversion of configuration |
|
Substrate Preference |
Tertiary > Secondary > Primary > Methyl |
Methyl > Primary > Secondary >> Tertiary |
|
Nucleophile Strength |
Weak nucleophile favored |
Strong nucleophile favored |
|
Leaving Group |
Good leaving group essential |
Good leaving group essential |
|
Solvent |
Polar protic solvents favor (stabilize carbocation) |
Polar aprotic solvents favor (no solvation of nucleophile) |
11. What is a carbene? Give one method of its generation.
Answer: A carbene is a neutral chemical species in which a carbon atom has two univalent substituents and two unshared electrons. It has a divalent carbon atom.
Generation: One common method is the α-elimination of HX from a haloalkane using a strong base: CHCl3+KOH⟶:CCl2+KCl+H2O (dichlorocarbene generation) Another method involves the photolysis or thermolysis of diazo compounds or ketenes.
12. Explain the concept of resonance. What are resonance structures and resonance hybrid?
Answer: Resonance is a phenomenon in which the actual structure of a molecule or ion is represented not by a single Lewis structure but by a combination of two or more Lewis structures, called resonance structures or contributing structures. These structures differ only in the arrangement of electrons (pi electrons or lone pairs), while the position of the nuclei remains the same. The resonance hybrid is the actual structure of the molecule, which is an average or composite of all the contributing resonance structures. It is more stable than any single resonance structure.
13. What is tautomerism? Give an example.
Answer: Tautomerism is a special type of isomerism in which two or more structural isomers exist in equilibrium and readily interconvert. This interconversion usually involves the migration of an atom or group (most commonly a hydrogen atom) and a change in the position of a double bond.
Example: Keto-enol tautomerism, where a ketone or aldehyde exists in equilibrium with its enol form (an alcohol with a double bond adjacent to the hydroxyl group): CH3COCH3⇌CH3C(OH)=CH2 (Acetone <0xE2><0x86><0x94> Enol form)
14. What are Diels-Alder reactions? Give the general reaction.
Answer: The Diels-Alder reaction is a [4+2] cycloaddition reaction between a conjugated diene (a molecule with two double bonds separated by a single bond) and a dienophile (an alkene or alkyne). This reaction forms a cyclic adduct, specifically a cyclohexene or its derivatives. It is a concerted, pericyclic reaction.
General Reaction: $$\begin{array}{c} \includegraphics[max width=2in]{diels_alder.png} \ Diene + Dienophile \longrightarrow Cycloadduct \end{array}$$
15. Explain the acidity order of carboxylic acids, phenols, and alcohols.
Answer: The acidity order is generally: Carboxylic acids > Phenols > Alcohols.
Carboxylic acids are the most acidic because their conjugate base, the carboxylate ion, is stabilized by resonance involving two equivalent oxygen atoms, effectively delocalizing the negative charge.
Phenols are more acidic than alcohols because their conjugate base, the phenoxide ion, is stabilized by resonance delocalization of the negative charge into the benzene ring.
Alcohols are the least acidic as their conjugate bases, alkoxides, have the negative charge localized on the oxygen atom with no significant resonance stabilization.
16. What is the Friedel-Crafts alkylation reaction? What are its limitations?
Answer: The Friedel-Crafts alkylation is an electrophilic aromatic substitution reaction in which an alkyl group is attached to an aromatic ring. It involves the reaction of an aromatic compound with an alkyl halide in the presence of a Lewis acid catalyst (e.g., AlCl3, FeCl3).
Limitations:
Polyalkylation: The introduction of an alkyl group activates the benzene ring towards further electrophilic substitution, leading to polyalkylation.
Rearrangement of carbocations: If the alkyl halide can form a more stable carbocation, rearrangement can occur, leading to products with different alkyl groups than the starting halide.
Reaction fails with deactivated rings: Aromatic rings with electron-withdrawing groups are deactivated and do not undergo Friedel-Crafts alkylation.
The catalyst can be deactivated: Amines and phenols can coordinate with the Lewis acid catalyst, deactivating it.
17. What is ozonolysis? What products are obtained from the ozonolysis of alkenes?
Answer: Ozonolysis is an organic reaction in which alkenes, alkynes, or azo compounds are cleaved with ozone (O3) to form smaller organic molecules containing carbonyl groups (aldehydes, ketones, or carboxylic acids).
Products from alkenes: The ozonolysis of an alkene typically yields aldehydes and/or ketones, depending on the substitution pattern of the alkene. A reductive workup (e.g., with zinc dust and acetic acid or dimethyl sulfide) preserves the carbonyl compounds, while an oxidative workup (e.g., with hydrogen peroxide) oxidizes aldehydes to carboxylic acids.
18. Explain the term "epoxide." How are epoxides synthesized?
Answer: An epoxide (or oxirane) is a cyclic ether with a three-membered ring containing one oxygen atom and two carbon atoms. The ring is approximately equilateral, and thus strained, making epoxides more reactive than acyclic ethers.
Synthesis: Epoxides are commonly synthesized by:
Reaction of alkenes with peroxyacids (e.g., mCPBA): This is a stereospecific syn addition of oxygen across the double bond.
Intramolecular Williamson ether synthesis: Reaction of a halohydrin (a molecule with a hydroxyl group and a halogen atom on adjacent carbons) with a base.
19. What is meant by the term "spectroscopy" in organic chemistry? Name two common spectroscopic techniques used for structure elucidation.
Answer: Spectroscopy in organic chemistry refers to the study of the interaction of electromagnetic radiation with organic molecules. By analyzing the resulting spectra, information about the structure, functional groups, and connectivity of atoms within the molecule can be obtained.
Two common techniques:
Nuclear Magnetic Resonance (NMR) Spectroscopy: Provides information about the number and types of hydrogen and carbon atoms in a molecule, their chemical environment, and their connectivity.
Infrared (IR) Spectroscopy: Provides information about the functional groups present in a molecule based on the absorption of specific frequencies of infrared light, which cause vibrations of molecular bonds.
20. What is a peptide bond? How is it formed?
Answer: A peptide bond (or amide bond) is a covalent chemical bond formed between two amino acid molecules when the carboxyl group of one amino acid reacts with the amino group of the other, releasing a molecule of water. This dehydration reaction links the amino acids together to form peptides and proteins.
21. What are carbohydrates? Give a classification with examples.
Answer: Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis. They are the most abundant organic molecules in nature and play crucial roles in energy storage and structural support.
Classification:
Monosaccharides: Simple sugars that cannot be hydrolyzed further (e.g., glucose, fructose, galactose).
Disaccharides: Composed of two monosaccharide units linked by a glycosidic bond (e.g., sucrose, lactose, maltose).
Oligosaccharides: Contain 3 to 10 monosaccharide units linked together.
Polysaccharides: Long chains of monosaccharide units (e.g., starch, cellulose, glycogen).
22. What are lipids? Give some examples of their functions.
Answer: Lipids are a broad group of naturally occurring molecules that are generally insoluble in water but soluble in nonpolar organic solvents. They are primarily composed of hydrocarbons.
Functions:
Energy storage: Triglycerides (fats and oils) are efficient energy storage molecules.
Structural components of cell membranes: Phospholipids and cholesterol are major components of biological membranes.
Hormones: Steroid hormones (e.g., testosterone, estrogen) regulate various physiological processes.
Insulation and protection: Fats provide thermal insulation and protect vital organs.
23. What are nucleic acids? What are their basic components?
Answer: Nucleic acids are biopolymers essential for all known forms of life. They carry the genetic information of an organism. The two main types are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
Basic components (nucleotides):
A nitrogenous base (adenine, guanine, cytosine, thymine in DNA; uracil in RNA).
A pentose sugar (deoxyribose in DNA; ribose in RNA).
A phosphate group.
24. Explain the concept of chirality. What is a chiral center?
Answer: Chirality refers to the property of a molecule or object that cannot be superimposed on its mirror image. A chiral molecule is non-superimposable on its mirror image.
Chiral center (stereocenter or asymmetric center): An atom, usually carbon, that is bonded to four different atoms or groups of atoms. The presence of one or more chiral centers is a common cause of chirality in organic molecules.
25. What is the difference between configuration and conformation?
Answer:
Configuration: Refers to the fixed spatial arrangement of atoms in a molecule that can only be changed by breaking and reforming covalent bonds. Isomers with different configurations are stereoisomers (e.g., enantiomers, diastereomers).
Conformation: Refers to the different spatial arrangements of atoms in a molecule that result from the rotation about single bonds. Conformers (rotamers) are different spatial arrangements that can interconvert without breaking any bonds.
