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Sindh Mdcat Exclusive Course Chemistry Carboxylic Acids And Derivatives — Solved Past Paper with Answers
All 20 MCQs from Sindh Mdcat Exclusive Course Chemistry Carboxylic Acids And Derivatives, solved with the correct answer highlighted and a full explanation for every question. This is a free MDCAT Sindh / DUHS past paper — no signup, no ads. Practise it interactively in timed mode, drill more with free MDCAT MCQs, or browse all Sindh / DUHS papers.
Attempt this paper interactively →Q1. Which of these in an aqueous solution of equal concentration has the lowest pH?
- A. Chloroethanoic acid✓
- B. Ethanoic acid
- C. Ethylamine
- D. Phenol
Explanation: The lowest pH corresponds to the strongest acid. Chloroethanoic acid is a stronger acid than ethanoic acid and phenol due to the electron-withdrawing inductive effect. of the chlorine atom. Ethylamine is a base and will have a pH above 7.
Why the other options are wrong
- B. Ethanoic acid (CH3COOH), or acetic acid, is a weak acid that partially dissociates in solution, thus releasing fewer H+ ions than chloroethanoic acid. This lower degree of dissociation leads to a higher pH compared to chloroethanoic acid.
- C. Ethylamine (CH3CH2NH2) is a weak base that, when dissolved in water, tends to accept protons to form the ethylammonium ion (CH3CH2NH3+). This reaction results in a higher pH, making it unsuitable for this question about the lowest pH.
- D. Phenol (C6H5OH) is considered a weak acid as it partially dissociates in water to release a limited number of H+ ions. However, its acidity is lower than that of chloroethanoic acid, leading to a relatively higher pH in solution.
Q2. Sodium Benzoate on reacting with soda lime forms:
- A. Benzoic Acid
- B. Benzene✓
- C. Toluene
- D. Benzaldehyde
Explanation: Sodium benzoate undergoes decarboxylation when heated with soda lime (NaOH + CaO). The -COONa group is removed as CO2, leaving behind toluene (C6H5CH3).This is a classic example of alkali-induced decarboxylation of a carboxylate salt.
Why the other options are wrong
- A. False. While benzoic acid is related to sodium benzoate, the reaction with soda lime does not produce benzoic acid. Instead, the decarboxylation process releases carbon dioxide and yields benzene.
- C. False. Toluene is not produced when sodium benzoate reacts with soda lime. The correct product from this reaction is benzene, which has a different structure than toluene.
- D. False. Benzaldehyde is not the product of this reaction. The decarboxylation of sodium benzoate yields benzene instead.
Q3. Which of the following reagents will reduce butanoic acid to butanol?
- A. LiAlH4✓
- B. NaBH4
- C. LiAlH4.H2O
- D. Both A and B
Explanation: LiAlH4 is a powerful reducing agent that can reduce. carboxylic acids to primary alcohols, such as converting butanoic acid to butanol, This occurs because LiAlH4 can donate hydrides that are capable of reducing the carboxyl. group. On the other hand, NaBH4 is not strong enough to reduce carboxylic acids; it is typically used for reducing aldehydes and ketones. LIAlH4 in the presence of water will decompose and cannot be used effectively for this reduction.
Why the other options are wrong
- B. NaBH4 is a milder reducing agent that is typically ineffective for reducing carboxylic acids to alcohols; it is more suited for aldehydes and ketones.
- C. The presence of water can lead to the decomposition of LiAlH4, which prevents it from being effective in reducing carboxylic acids.
- D. This option is incorrect because while LiAlH4 is effective, NaBH4 does not have the necessary strength to reduce carboxylic acids.
Q4. Carboxylic acids can be reduced to corresponding alcohols. Which of the following reagent can be used for this purpose?
- A. KMnO4
- B. LiAlH4✓
- C. K2Cr2O4
- D. H2SO4
Explanation: For this reaction to take place, you need the proper reducing agent, which is only LiAlH4 i.e. option B. (NOTE: NaBH4 is not a strong enough reducing agent to reduce carboxylic acids, but LiAlH4 is.)Options A, C, and D; all describe OXIDISING agents.
Why the other options are wrong
- A. Options A, C, and D all describe OXIDISING agents.Oxidizing agents, also known as oxidants, are substances that facilitate oxidation reactions by accepting electrons from other substances. In the context of redox reactions, oxidation refers to the loss of electrons from a species, resulting in an increase in its oxidation state. The oxidizing agent, on the other hand, gains those electrons, leading to a decrease in its own oxidation state.
- C. Options A, C, and D all describe OXIDISING agents.Oxidizing agents, also known as oxidants, are substances that facilitate oxidation reactions by accepting electrons from other substances. In the context of redox reactions, oxidation refers to the loss of electrons from a species, resulting in an increase in its oxidation state. The oxidizing agent, on the other hand, gains those electrons, leading to a decrease in its own oxidation state.
- D. Options A, C, and D; all describe OXIDISING agents.Oxidizing agents, also known as oxidants, are substances that facilitate oxidation reactions by accepting electrons from other substances. In the context of redox reactions, oxidation refers to the loss of electrons from a species, resulting in an increase in its oxidation state. The oxidizing agent, on the other hand, gains those electrons, leading to a decrease in its own oxidation state.
Q5. Which of the following carboxylic acids will be the most acidic?
- A. CH3CHClCH2COOH
- B. CH3CH2CCl2COOH✓
- C. CH3CH2CHClCOOH
- D. CH3CH2CH2COOH
Explanation: The acidity of carboxylic acids increases with the presence of electron-withdrawing groups (like-CI) near the -COOH group. CH3CH2CCI2COOH has two chlorine atoms on the B-carbon, which strongly stabilize the carboxylate ion through the inductive effect. Fewer or more distant chlorines (as in the other options) make the acid less acidic.
Why the other options are wrong
- A. This carboxylic acid contains one chlorine atom, which exerts an electron-withdrawing effect. However, it is less effective in stabilizing the carboxylate anion compared to options with more electronegative groups or closer proximity to the carboxyl group.
- C. This carboxylic acid has one chlorine atom, but it is farther from the carboxyl group compared to the correct option. Thus, it is less acidic than option B due to weaker stabilization of the carboxylate anion.
- D. This option contains no electronegative groups, making it the least acidic of all the choices. The lack of electron-withdrawing effects means that the carboxylate anion is not stabilized, resulting in the lowest acidity.
Q6. An ester is prepared by the reaction of:
- A. Two alcohols
- B. Carboxylic acid and alcohol✓
- C. Ketone and alcohol
- D. Aldehyde and alcohol
- E. All of these
Explanation: When a carboxylic acid is treated with an alcohol and an acid catalyst, an ester is formed (along with water):
Why the other options are wrong
- A. It is possible for two alcohol molecules to dehydrate each other under the right conditions but it doesn't produce ester. The hydrogen atom of one molecule OH group is removed, while only the hydrogen atom of the second molecule's OH group is removed.
- C. Aldehydes and ketones react with two moles of an alcohol to give 1,1-geminal diethers more commonly known as acetals. The term "acetal' used to be restricted to systems derived from aldehydes and the term "ketal" applied to those from ketones, but chemists now use acetal to describe both.
- D. Aldehydes and ketones react with two moles of an alcohol to give 1,1-geminal diethers more commonly known as acetals. The term "acetal' used to be restricted to systems derived from aldehydes and the term "ketal" applied to those from ketones, but chemists now use acetal to describe both.
- E. Wrong answer.B is the right option.
Q7. Which of the following is the strongest acid?
- A. Propanoic acid
- B. Fluoroethanoic acid
- C. Trichloroethanoic acid✓
- D. Nitroethanoic acid
Explanation: Trichloroacetic Acid (TCA) is a much stronger acid than all the acids mentioned in the options, from a chemical ionization standpoint, because the electronegative Chlorine atoms draw electron density away from the carboxyl end of the molecule, creating a partial positive charge on the carboxyl group, and allowing easier removal of the positively charged Hydrogen ion, H+.
Why the other options are wrong
- A. Propanoic acid (propionic acid) is weaker than trichloroethanoic acid.
- B. Fluoroethanoic acid (fluoroacetic acid) is weaker than trichloroethanoic acid.
- D. Nitroethanoic acid (nitroacetic acid) is weaker than trichloroethanoic acid.
Q8. Hydrolysis of Nitriles produces:
- A. Carboxylic acid✓
- B. Aldehydes
- C. Ketones
- D. Esters
Explanation: The hydrolysis of nitriles, which are organic molecules containing a cyano group, leads to carboxylic acid formation.An example is given below:
Why the other options are wrong
- B. Oxidation of –COOH further can form aldehyde and ketone. But it is not formed by hydrolysis of nitriles.
- C. Oxidation of –COOH further can form aldehyde and ketone. But it is not formed by hydrolysis of nitriles.
- D. Esters are formed when the carboxylic acid is heated with the alcohol in the presence of a catalyst. In this reaction, concentrated sulphuric acid is used as a catalyst; the dry form of hydrogen chloride gas is used in some cases. This method of reaction is used to convert alcohol into an ester.
Q9. Acetic anhydride is a product of acetic acid, as a result of the following reaction:
- A. Dehydration✓
- B. Reduction
- C. Oxidation
- D. Esterification
Explanation: Acetic anhydride can be prepared by the dehydration of acetic acid at 800°C so, option A is correct.The reduction of acetic acid doesn’t give acetic anhydride, it produces ethanol.Oxidation of acetic acid produces various by-products such as carbon monoxide and carbon dioxide.Esterification of acetic acid produces various esters.
Why the other options are wrong
- B. Reduction of acetic acid gives ethanol. The OH group is the main site of reaction, as illustrated by the conversion of acetic acid to acetyl check loride.
- C. Acetic acid can be further oxidized to carbon dioxide and water. Acetic acid is oxidized by oxygen in the presence of UV light, TiO2 and dissolved copper ions being the catalysis. The reaction rate is zeroth order with respect both to acetic acid and dissolved oxygen, and is proportional to the absorbed light intensity, with a quantum yield of 0.06.
- D. Esters have also been prepared by the passing of the vapors of acetic acid and alcohol over a heated catalyst.
Q10. Reagent used to reduce a carboxylic acid to an alkane is:
- A. Ni / H2
- B. NaBH4
- C. P / HI✓
- D. LiAlH4
Explanation: The reduction of a carboxylic acid to an alkane typically requires a strong reducing agent. One commonly used reagent for this purpose is lithium aluminum hydride (LiAlH₄). LiAlH₄ is a powerful reducing agent that can selectively reduce the carbonyl group of a carboxylic acid to a primary alcohol, and further reduction with additional equivalents of LiAlH₄ can convert the alcohol to an alkane. Other reducing agents, such as sodium borohydride (NaBH₄), are generally not strong enough to directly reduce a carboxylic acid to an alkane.Hot red Phosphorus and Hydrogen Iodide at 200℃ is used to reduce carboxylic acids to alkanes according to the equation below, using propionic acid as an example:C2H5COOH + 6HI —---------------------> C3H8 + 2H2O + 3I2Therefore option C is correct.
Why the other options are wrong
- A. Option A is incorrect because Ni/H2 are mild reducing reagents, commonly used for reducing alkenes to alkanes.
- B. Option B is incorrect because NaBH4 is a mild reducing agent that is commonly used for reducing aldehydes and ketones to primary and secondary alcohols respectively.
- D. Option D is incorrect because LiAlH4 is a strong reducing agent that reduces carboxylic acids to primary alcohols.
Q11. What happens if we react a carboxylic acid with an acyl chloride?
- A. It forms an acid anhydride✓
- B. It forms an acyl halide
- C. It does not react due to resonance
- D. It does not react due to bad leaving group
Explanation: When a carboxylic acid reacts with an acyl chloride, it forms an acid anhydride through the elimination of HCl. The general reaction can be represented as:RCOCl + R'COOH → RCOOCOR' + HCl
Why the other options are wrong
- B. This option is not accurate. The reaction between a carboxylic acid and an acyl chloride does not typically result in the formation of an acyl halide. Acyl chlorides already contain a halogen atom (-Cl) in the acyl group, so there is no need for halogen substitution in this reaction.
- C. It does not react due to resonance.This option is not accurate. Carboxylic acids can react with acyl chlorides, and the presence of resonance in carboxylic acids does not prevent the reaction.
- D. This option is not accurate. Acyl chlorides are highly reactive compounds and have good leaving groups (usually halogens such as chlorine or bromine). They can readily react with carboxylic acids, forming new compounds.
Q12. What is common when carboxylic acid is reacted with base or with alcohol?
- A. Catalyst
- B. Formation of ester
- C. Formation of salt
- D. Formation of water✓
Explanation: When a carboxylic acid is reacted with a base, such as sodium hydroxide, it undergoes neutralization to form salt and water. For example, the reaction between acetic acid and sodium hydroxide produces sodium acetate and water:CH3COOH + NaOH → CH3COO−Na+ + H2OWhen a carboxylic acid is reacted with an alcohol in the presence of a catalyst, such as concentrated sulfuric acid, it undergoes esterification to form an ester and water. For example, the reaction between acetic acid and ethanol produces ethyl acetate and water:CH3COOH + C2H5OH → CH3COOC2H5 + H2OTherefore, the formation of water is a common feature in both reactions.
Why the other options are wrong
- A. In some cases, a catalyst may be used to facilitate the reaction between a carboxylic acid and a base or alcohol, but it is not a common requirement. Therefore, this option is not the correct answer.
- B. While carboxylic acids can react with alcohols to form esters in a different type of reaction called esterification, this is not the common result when reacting specifically with a base. Therefore, this option is not the correct answer.
- C. When a carboxylic acid reacts with a base, it forms a salt known as carboxylate. However, this is not the common result when reacting with alcohol. Therefore, this option is not the correct answer.
Q13. HCOOH is the structure of:
- A. Acetic acid
- B. Formic acid✓
- C. Valeric acid
- D. Caproic acid
Explanation: Formic acid is a colorless, fuming liquid with a pungent acrid odor with the chemical formula HCOOH. Formic acid is systematically named as methanoic acid.
Why the other options are wrong
- A. Given compound has one carbon while acetic acid has two carbon atoms.
- C. Given compound has one carbon while valeric acid has 5 carbon atoms.
- D. Given compound has one carbon while caproic acid has 6 carbon atoms.
Q14. Which of the following is not the physical property of acetic acid?
- A. It is a colorless liquid with a boiling point of 118° C.
- B. It is miscible with water, alcohol and ether in all proportions.
- C. It is used in the manufacture of pickles.
- D. Takes part in the metabolism of one-carbon compounds.✓
Explanation: Ethanoic acid, often known as acetic acid, has a strong vinegar odor and a sour flavor. It is a colorless, transparent liquid. It is also used in the manufacture of pickles.Folate is a key element that takes part in the one-carbon metabolism (a series of interlinking metabolic pathways that provides 1C units (methyl groups) for the synthesis of DNA, polyamines, amino acids, creatine, and phospholipids).
Why the other options are wrong
- A. This is a correct fact that acetic acid is a colorless liquid with boiling point of 118° C.
- B. This is also true for acetic acid that it dissolves readily in water, alcohol, and ethers.
- C. Acetic acid commonly known as vinegar is used in food preservation such as pickles.
Q15. Solubility of carboxylic acids:
- A. Decreases with increase of volume
- B. Decreases with increase in molecular mass✓
- C. Increases with increase in molecular mass
- D. None of these
Explanation: As the carbon chain increases, the polar character decreases and the non polar character increases causing a decrease in solubility of the carboxylic acids.
Why the other options are wrong
- A. False. The statement is misleading as 'volume' is not directly correlated with the molecular structure influencing solubility. The correct factor is the carbon chain length.
- C. False. This contradicts the established trend where increased carbon chain length reduces polarity, leading to lower solubility in polar solvents like water.
- D. False. This option is incorrect as option B accurately describes the trend in solubility with molecular mass.
Q16. Which type of carboxylic acid is produced from the hydrolysis of nitriles?
- A. Beta-hydroxy carboxylic acids
- B. Acids having one extra no of carbon atom✓
- C. Acids having one carbon less than the starting material
- D. Alpha-hydroxy acids
Explanation: Hydrolysis of nitriles produces a carboxylic acid with one extra carbon. This is an important reaction in organic chemistry as it elongates the carbon chain.
Why the other options are wrong
- A. False.Hydrolysis of nitriles produces a carboxylic acid with one extra carbon. This is an important reaction in organic chemistry as it elongates the carbon chain.
- C. False.Hydrolysis of nitriles produces a carboxylic acid with one extra carbon. This is an important reaction in organic chemistry as it elongates the carbon chain.
- D. False.Hydrolysis of nitriles produces a carboxylic acid with one extra carbon. This is an important reaction in organic chemistry as it elongates the carbon chain.
Q17. In IUPAC nomenclature, Carboxylic acids are named as_?
- A. Alkoxy acid
- B. Alkanoic acid✓
- C. Alkyl carboxylic acid
- D. Alkoxylate acid
Explanation: In IUPAC nomenclature, carboxylic acids are named as alkanoic acid.
Why the other options are wrong
- A. Option A) Alkoxy acid refers to compounds with an alkoxy group (-OR) attached to a carbon atom, not a carboxylic acid group (-COOH).
- C. Option C) Alkyl carboxylic acid is not a commonly used term in IUPAC nomenclature. Carboxylic acids are generally referred to as alkanoic acids.
- D. Option D) Alkoxylate acid is not a recognized term in IUPAC nomenclature. The term alkoxylate typically refers to a compound containing an alkoxide group (-OR), not a carboxylic acid group.
Q18. Maleic and Fumaric acid, both have chemical C4H4O4. The structure of these acids is shown below. What type of isomers are these?
- A. Cis-trans isomers✓
- B. Structural isomers
- C. Metamers
- D. Position Isomers
Explanation: Maleic acid and fumaric acid are cis-trans (geometrical) isomers.They have the same molecular formula (C4H4O4) but differ in the arrangement of -COOH groups aroundthe C=C bond: cis in maleic acid, trans in fumaric acid.
Why the other options are wrong
- B. This option is incorrect. Structural isomers have different connectivity of atoms, which is not the case for maleic and fumaric acids as they share the same molecular formula (C4H4O4) and connectivity, differing only in the spatial arrangement of groups.
- C. This option is also incorrect. Metamers are isomers that differ in the alkyl groups on either side of a functional group. Maleic and fumaric acids do not fit this definition as they both contain the same functional groups and similar alkyl arrangements.
- D. This option is incorrect as well. Position isomers differ in the position of functional groups or double bonds within the same carbon chain. Since both maleic and fumaric acids have the double bond in the same location, they cannot be classified as position isomers.
Q19. Carboxylic acids do not give the characteristics reaction of >C=O group because of:
- A. Polar nature
- B. Attached alkyl group
- C. Resonance✓
- D. Symmetrical structure
Explanation: The carbonyl group of ketones & aldehydes is more. electrophilic than in carboxylic acids. This is because the lone pairs on the Oxygen atom attached to a Hydrogen atom in the CO2H are involved in 'resonance', thereby making the carbon atom less electrophilic.
Why the other options are wrong
- A. The polar nature of carboxylic acids enhances their solubility in polar solvents like water. However, it does not explain the unique reactivity of the carbonyl group in carboxylic acids compared to aldehydes and ketones. Therefore, this option is not correct.
- B. While the attached alkyl group can influence the overall electronic environment of the carboxylic acid, it does not primarily account for the difference in reactivity of the carbonyl group. It is the interplay of resonance stabilization and the presence of other functional groups that plays a more significant role.
- D. Although the symmetrical structure may influence certain physical properties of carboxylic acids, it does not fundamentally explain why these compounds do not exhibit the characteristic reactions typical of carbonyl groups in other organic compounds. Thus, this option is incorrect.
Q20. Which of the following carboxylic acids is used in medicine as a local irritant?
- A. Formic acid✓
- B. Acetic acid
- C. Benzoic acid
- D. Amino acid
Explanation: The carboxylic acid used in medicine as a local irritant is Formic acid (HCOOH).Formic acid (also found in ant stings) causes redness and irritation on skin.In medicine, dilute formic acid preparations are sometimes used as counter-irritants to relieve pain in muscles and joints.
Why the other options are wrong
- B. Main use in medicine: as vinegar (dilute acetic acid), mild antiseptic, food preservative.In concentrated form (glacial acetic acid), it is corrosive and can irritate skin, but it is not normally used as a local irritant in medicine.Its main pharmaceutical use is in acetates and as a solvent, not specifically as a therapeutic counter-irritant
- C. It is used in carbonated and still beverages, syrups, fruit salads, icings, jams, jellies, preserves, salted margarine, mincemeat, pickles and relishes, pie, pastry fillings, prepared salads, fruit cocktai, soy sauce, and caviar.
- D. Amino acids are building blocks of proteins and are not typically used as local irritants or antiseptics in medicine.
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