Questions: Organic Chemistry basics
Organic Chemistry – Some Basic Principles and Techniques22 min readLesson 6 of 7
Q1. Why is carbon tetravalent, and what shapes does it give?
Answer: Carbon has four electrons in its valence shell and needs four more to complete its octet, so it forms four covalent bonds. The four bonds are arranged according to the hybridisation of the carbon: four single bonds give a tetrahedral shape with bond angles near 109.5 degrees, a double bond gives a trigonal planar shape with angles near 120 degrees, and a triple bond gives a linear shape with angles near 180 degrees.
Hinglish: Carbon ke paas chaar valence electrons hain, isliye chaar covalent bonds banata hai. Chaar single bonds tetrahedral shape dete hain, double bond planar 120° aur triple bond linear 180° geometry deta hai.
Q2. What is the difference between a sigma bond and a pi bond?
Answer: A sigma bond is formed by the head-on overlap of orbitals along the line joining the nuclei, and it is strong and allows free rotation. A pi bond is formed by the sideways overlap of p orbitals above and below the internuclear axis, and it is weaker and restricts rotation. A double bond has one sigma and one pi bond, and a triple bond has one sigma and two pi bonds.
Hinglish: Sigma bond head-on overlap se banti hai, strong hoti hai aur rotation allow karti hai. Pi bond sideways overlap se banti hai, kamzor hoti hai aur rotation nahi hoti. Double bond mein ek sigma aur ek pi hoti hai, triple mein ek sigma aur do pi.
Q3. Write the IUPAC name of CH3-CH(CH3)-CH2-CH2-OH.
Answer: The longest chain has four carbons and carries the hydroxyl group, so the root is butanol. The hydroxyl must get the lowest locant, so numbering starts from the end near it and the group is at position one, giving butan-1-ol. The methyl substituent is at position three, so the full name is 3-methylbutan-1-ol.
Hinglish: Chain mein chaar carbon hain aur hydroxyl group ki priority hai, isliye root butanol hai. Hydroxyl ko lowest number dene ke liye numbering us taraf se karte hain, isliye butan-1-ol. Methyl carbon 3 par hai, to naam 3-methylbutan-1-ol.
Q4. What is the order of priority of functional groups for IUPAC naming?
Answer: The principal group is chosen by priority, and the suffix is taken from the highest priority group present. The common order is carboxylic acid, then aldehyde, then ketone, then alcohol, then amine, and finally the alkene and alkyne bonds. Lower priority groups are written as prefixes. The priority order is fixed and does not depend on which group is present in larger amount.
Hinglish: Suffix principal group se liya jaata hai, aur priority order hai carboxylic acid, aldehyde, ketone, alcohol, amine, phir alkene aur alkyne. Lower priority groups prefix ban jaate hain. Yeh order fixed hai aur amount par depend nahi karta.
Q5. Distinguish between chain, position and functional group isomerism.
Answer: Chain isomers have the same molecular formula but different carbon skeletons, as in butane and 2-methylpropane. Position isomers have the same skeleton and functional group but the group sits at a different position on the chain. Functional group isomers have the same molecular formula but different functional groups, as in ethanol and dimethyl ether, or in pentanoic acid and ethyl propanoate. All three are structural isomerisms because the connectivity differs.
Hinglish: Chain isomers mein skeleton alag hota hai (butane aur 2-methylpropane). Position isomers mein functional group ki position alag hoti hai. Functional group isomers mein functional group hi alag hota hai (ethanol aur dimethyl ether). Teeno structural isomerism hain kyunki connectivity alag hoti hai.
Q6. Explain the inductive effect with the acidity of substituted acetic acids.
Answer: The inductive effect is the displacement of sigma bond electrons toward a more electronegative atom. A group with a minus I effect withdraws electron density, stabilises the negative charge of the carboxylate ion and increases acidity. A group with a plus I effect pushes electrons toward the carboxylate and destabilises it, decreasing acidity. This is why trichloroacetic acid is stronger than acetic acid, and why alkyl substituted acids are weaker.
Hinglish: Inductive effect sigma bond ke electrons ka displacement hai. -I group carboxylate ki negative charge ko stabilise karke acidity badhata hai, aur +I group destabilise karke acidity ghatata hai. Isliye trichloroacetic acid acetic acid se strong hai.
Q7. Why is a tertiary carbocation more stable than a primary carbocation?
Answer: Alkyl groups release electron density through the inductive effect and stabilise the positively charged carbon. Hyperconjugation also operates, because the adjacent carbon-hydrogen bonds can overlap with the empty p orbital and spread the charge. A tertiary carbocation has three alkyl groups and therefore more such stabilising interactions than a primary one, so it is lower in energy and easier to form.
Hinglish: Alkyl groups electron density release karke positive charge ko stabilise karte hain, aur hyperconjugation se charge fail jaata hai. Tertiary carbocation par teen alkyl groups hote hain, isliye woh primary se zyada stable hota hai.
Q8. Which distillation method would you use for a liquid that decomposes at its boiling point?
Answer: Distillation under reduced pressure is used. Lowering the pressure lowers the boiling point of the liquid, so it can be distilled at a temperature below the temperature at which it decomposes. Heating the liquid at atmospheric pressure would destroy it before it distilled, so the reduced pressure method protects the compound while still separating it from impurities.
Hinglish: Distillation under reduced pressure use karte hain. Pressure kam karne se boiling point kam ho jaata hai, isliye liquid decompose hone wale temperature se neeche distil ho jaata hai. Normal pressure par garam karne se compound kharab ho jaata.
Q9. How is nitrogen estimated by the Kjeldahl method?
Answer: The compound is heated with concentrated sulphuric acid, which converts the nitrogen into ammonium sulphate. The mixture is then made alkaline, and the ammonia released is distilled into a known volume of a standard acid. The unreacted acid is titrated with a standard base, and the difference gives the ammonia produced. The mass of nitrogen follows from the moles of ammonia, and the percentage of nitrogen follows from the mass of the sample.
Hinglish: Compound ko concentrated sulphuric acid ke saath garam karte hain, jisse nitrogen ammonium sulphate ban jaata hai. Phir alkali daalkar ammonia distill karte hain aur known acid mein collect karke unreacted acid ko base se titrate karte hain. Difference se ammonia ke moles aur nitrogen ka percentage nikalta hai.
Q10. A compound contains 40 percent carbon, 6.7 percent hydrogen and the rest oxygen. Find its empirical formula.
Answer: Divide each percentage by the atomic mass: carbon 40 over 12 gives 3.33, hydrogen 6.7 over 1 gives 6.7 and oxygen 53.3 over 16 gives 3.33. Divide all by the smallest value 3.33 to get 1, 2 and 1, so the empirical formula is CH2O. The molecular formula is a multiple of this and is found from the molar mass, so if the molar mass is 180 the molecular formula is C6H12O6.
Hinglish: Har percentage ko atomic mass se divide karo: C ke liye 3.33, H ke liye 6.7, O ke liye 3.33. Sabse chhote value se divide karke 1 : 2 : 1 milta hai, isliye empirical formula CH2O. Molar mass 180 ho to molecular formula C6H12O6 hoga.
Quiz below: five questions on hybridisation, IUPAC naming, isomerism, electronic effects and purification. Attempt them before rereading the formula sheet.