1. An amine is best described as:
ⓐ. an organic derivative of methane formed by replacing carbon atoms with nitrogen
ⓑ. an ammonia derivative formed by replacing hydrogen with alkyl or aryl groups
ⓒ. an ammonium salt formed only when ammonia reacts with a strong mineral acid
ⓓ. a nitrogen compound in which nitrogen must be part of a ring
Correct Answer: an ammonia derivative formed by replacing hydrogen with alkyl or aryl groups
Explanation: Ammonia contains three hydrogen atoms bonded to nitrogen. Replacement of one or more of these hydrogen atoms by alkyl or aryl groups produces an amine. Replacement may occur once, twice, or three times, giving different general amine formulas. The nitrogen atom remains bonded to carbon-containing groups while retaining its characteristic electron pair in a neutral amine. An ammonium salt is produced by protonation and is not the basic definition of an amine.
2. For an ordinary neutral tertiary amine, the blank is filled by ______.
\[
\mathrm{General\ formula}=\_\_\_\_
\]
ⓐ. \(\mathrm{RNH_2}\)
ⓑ. \(\mathrm{R_2NH}\)
ⓒ. \(\mathrm{R_3N}\)
ⓓ. \(\mathrm{R_4N^+}\)
Correct Answer: \(\mathrm{R_3N}\)
Explanation: A tertiary amine contains three carbon groups bonded directly to nitrogen. Since all three hydrogen atoms of ammonia have been replaced, no nitrogen-bonded hydrogen remains. Its neutral general notation is therefore \(\mathrm{R_3N}\). The species \(\mathrm{R_4N^+}\) has a fourth carbon group and carries a positive charge. The absence of an \(\mathrm{N-H}\) bond does not remove the lone pair from a neutral tertiary amine.
3. Among the following compounds, the one that is not an amine is:
ⓐ. methylamine, \(\mathrm{CH_3NH_2}\)
ⓑ. nitrobenzene, \(\mathrm{C_6H_5NO_2}\)
ⓒ. ethylamine, \(\mathrm{C_2H_5NH_2}\)
ⓓ. benzenamine (aniline), \(\mathrm{C_6H_5NH_2}\)
Correct Answer: nitrobenzene, \(\mathrm{C_6H_5NO_2}\)
Explanation: Methylamine, ethylamine, and aniline contain nitrogen in an amino group derived structurally from ammonia. Nitrobenzene instead contains the nitro group \(\mathrm{-NO_2}\). The presence of nitrogen alone is not enough to classify a compound as an amine. Nitrogen may occur in several other functional groups, including nitro groups, amides, nitriles, and diazonium groups. Functional-group classification depends on the bonding environment of nitrogen rather than only on the molecular formula.
4. The characteristic basic and nucleophilic behaviour of a neutral amine arises mainly because the nitrogen lone pair:
ⓐ. is used only to form the existing nitrogen–carbon bonds
ⓑ. prevents nitrogen from approaching electron-deficient species
ⓒ. makes the amine permanently positively charged
ⓓ. can accept a proton or attack an electron-deficient centre
Correct Answer: can accept a proton or attack an electron-deficient centre
Explanation: The nitrogen atom of a neutral amine possesses a non-bonding electron pair. This pair can bind a proton, allowing the amine to behave as a base. It can also be donated toward an electron-deficient atom during a nucleophilic reaction. A neutral amine is not permanently charged; positive charge generally appears after protonation or formation of a fourth bond. Any factor that reduces lone-pair availability can weaken both basic and nucleophilic behaviour.
5. Match each familiar compound in Column I with its structural pattern in Column II.
| Column I | Column II |
|---|
| P. Methylamine | 1. \(\mathrm{ArNH_2}\) |
| Q. Dimethylamine | 2. \(\mathrm{R_2NH}\) |
| R. Trimethylamine | 3. \(\mathrm{RNH_2}\), where \(\mathrm{R=CH_3}\) |
| S. Aniline | 4. \(\mathrm{R_3N}\) |
ⓐ. P-3, Q-2, R-4, S-1
ⓑ. P-2, Q-3, R-1, S-4
ⓒ. P-3, Q-4, R-2, S-1
ⓓ. P-1, Q-2, R-4, S-3
Correct Answer: P-3, Q-2, R-4, S-1
Explanation: Methylamine is \(\mathrm{CH_3NH_2}\), so it matches the \(\mathrm{RNH_2}\) pattern with \(\mathrm{R=CH_3}\). Dimethylamine contains two methyl groups and matches \(\mathrm{R_2NH}\). Trimethylamine contains three methyl groups and matches \(\mathrm{R_3N}\). Aniline contains an aryl group directly bonded to \(\mathrm{NH_2}\), so it matches \(\mathrm{ArNH_2}\). The prefixes methyl, dimethyl, and trimethyl reveal the number of methyl groups attached to nitrogen.
6. A neutral nitrogen atom in an ordinary amine is most consistently described as having:
ⓐ. three valence electrons, two sigma bonds, and one lone pair
ⓑ. five valence electrons, four sigma bonds, and no formal charge
ⓒ. seven valence electrons, two sigma bonds, and two lone pairs
ⓓ. five valence electrons, three sigma bonds, and one lone pair
Correct Answer: five valence electrons, three sigma bonds, and one lone pair
Explanation: Nitrogen belongs to Group 15 and has five valence electrons. In an ordinary neutral amine, it forms three sigma bonds using three of these electrons. The remaining two electrons form one lone pair. Formation of four ordinary covalent bonds without a lone pair generally gives nitrogen a positive formal charge. The three-bond-and-one-lone-pair description explains why neutral amines can donate an electron pair.
7. In the Brønsted–Lowry description, an amine behaves as a base when it:
ⓐ. releases an electron into the surrounding medium
ⓑ. donates a proton to an alkyl group
ⓒ. accepts a proton through the nitrogen lone pair
ⓓ. produces a proton by breaking a carbon–hydrogen bond
Correct Answer: accepts a proton through the nitrogen lone pair
Explanation: A Brønsted–Lowry base is defined as a proton acceptor. An amine accepts \(\mathrm{H^+}\) at nitrogen because the lone pair can form a bond with the proton. The resulting species is a substituted ammonium ion. Proton donation would describe acidic behaviour rather than the usual basic behaviour of an amine. This definition concerns proton transfer specifically, whereas the Lewis definition applies more broadly to electron-pair donation.
8. From the Lewis acid–base viewpoint, a neutral amine is classified as a Lewis base because it:
ⓐ. donates an electron pair to form a bond
ⓑ. accepts an electron pair into the nitrogen lone pair
ⓒ. releases hydroxide ions without interacting with another species
ⓓ. donates a nitrogen-bonded proton to an electron-rich species
Correct Answer: donates an electron pair to form a bond
Explanation: A Lewis base is an electron-pair donor. The non-bonding pair on amine nitrogen can be donated to an electron-deficient species. The receiving species acts as the Lewis acid. Protonation is one example because \(\mathrm{H^+}\) accepts the nitrogen electron pair, but Lewis-base behaviour is not restricted to proton transfer. Describing the amine as an electron-pair acceptor reverses the donor–acceptor roles.
9. A reaction centre contains an amine nitrogen with an available lone pair and a positively polarised carbon atom. Their most likely roles are:
ⓐ. the amine nitrogen is an electrophile, and the carbon atom is a nucleophile
ⓑ. both centres are electrophiles because both participate in bond formation
ⓒ. both centres are nucleophiles because both contain valence electrons
ⓓ. the amine nitrogen is a nucleophile, and the carbon atom is an electrophile
Correct Answer: the amine nitrogen is a nucleophile, and the carbon atom is an electrophile
Explanation: A nucleophile supplies an electron pair during bond formation. The available lone pair makes the amine nitrogen electron-rich and capable of attacking another centre. A positively polarised carbon atom is electron-deficient and can accept that pair, so it acts as an electrophile. Bond formation results from attraction between these complementary centres. Merely possessing valence electrons does not make every atom a nucleophile; the availability and distribution of electron density matter.
10. The symbols \(\mathrm{R}\) and \(\mathrm{Ar}\) used in amine formulas generally represent:
ⓐ. \(\mathrm{R}\) as an aryl group and \(\mathrm{Ar}\) as an alkyl group
ⓑ. \(\mathrm{R}\) as a proton and \(\mathrm{Ar}\) as an ammonium ion
ⓒ. \(\mathrm{R}\) as an alkyl group and \(\mathrm{Ar}\) as an aryl group
ⓓ. \(\mathrm{R}\) as a carbonyl group and \(\mathrm{Ar}\) as a nitro group
Correct Answer: \(\mathrm{R}\) as an alkyl group and \(\mathrm{Ar}\) as an aryl group
Explanation: The symbol \(\mathrm{R}\) is commonly used for an alkyl group or a general carbon-containing residue. The symbol \(\mathrm{Ar}\) specifically denotes an aryl group derived from an aromatic ring. Thus, \(\mathrm{RNH_2}\) commonly represents an alkylamine pattern, while \(\mathrm{ArNH_2}\) represents an arylamine pattern. The symbols do not independently show the exact number of carbon atoms present. Their distinction becomes especially useful when comparing aniline-like compounds with ordinary alkylamines.
11. Protonation of dimethylamine produces:
ⓐ. \(\mathrm{(CH_3)_2NH_2^+}\)
ⓑ. \(\mathrm{CH_3CH_2NH_3^+}\)
ⓒ. \(\mathrm{(CH_3)_3NH^+}\)
ⓓ. \(\mathrm{CH_3NH_3^+}\)
Correct Answer: \(\mathrm{(CH_3)_2NH_2^+}\)
Explanation: Dimethylamine has the formula \(\mathrm{(CH_3)_2NH}\). Protonation adds \(\mathrm{H^+}\) to the nitrogen lone pair without changing the two methyl groups. The product therefore contains two methyl groups, two hydrogen atoms bonded to nitrogen, and an overall positive charge. The expression \(\mathrm{(CH_3)_3NH^+}\) would contain three methyl groups and would arise from protonation of trimethylamine. Protonation changes the charge and hydrogen count at nitrogen, not the identity of the attached alkyl groups.
12. During protonation of a neutral amine, the donor–acceptor roles are described accurately by:
ⓐ. The amine accepts an electron pair from \(\mathrm{H^+}\) and acts as a Lewis acid
ⓑ. \(\mathrm{H^+}\) donates an electron pair to nitrogen and acts as a Lewis base
ⓒ. The amine donates a proton to \(\mathrm{H^+}\) without using its nitrogen lone pair
ⓓ. The amine donates its nitrogen lone pair to \(\mathrm{H^+}\), forming a coordinate bond
Correct Answer: The amine donates its nitrogen lone pair to \(\mathrm{H^+}\), forming a coordinate bond
Explanation: A neutral amine possesses a non-bonding electron pair on nitrogen. During protonation, nitrogen supplies both electrons needed for the new bond to \(\mathrm{H^+}\). The amine is therefore the Lewis base and electron-pair donor, while the proton is the Lewis acid and electron-pair acceptor. The bond is described as coordinate at the moment of formation because both bonding electrons originate from nitrogen. Protonation does not make the amine an electron-pair acceptor.
13. Trimethylamine forms an adduct with electron-deficient \(\mathrm{BF_3}\). In this interaction:
ⓐ. boron donates a lone pair to nitrogen, so \(\mathrm{BF_3}\) is the Lewis base
ⓑ. nitrogen donates its lone pair to boron, so trimethylamine is the Lewis base
ⓒ. nitrogen accepts an electron pair from boron, so trimethylamine is the electrophile
ⓓ. a proton is transferred from trimethylamine to \(\mathrm{BF_3}\)
Correct Answer: nitrogen donates its lone pair to boron, so trimethylamine is the Lewis base
Explanation: Trimethylamine contains an available lone pair on nitrogen. Boron in \(\mathrm{BF_3}\) has an incomplete octet and can accept an electron pair. Nitrogen therefore donates its lone pair to boron to form a coordinate bond. The amine acts as the Lewis base and nucleophile, while \(\mathrm{BF_3}\) acts as the Lewis acid and electrophile. No proton transfer is required because this interaction is explained directly by electron-pair donation and acceptance.
14. Use the arrangement described below. A central nitrogen atom is bonded to a methyl group, an ethyl group, and one hydrogen atom, and it also possesses one lone pair. This structure represents:
ⓐ. a primary amine because one hydrogen is attached to nitrogen
ⓑ. a secondary amine because two carbon groups are attached to nitrogen
ⓒ. a tertiary amine because three different substituents surround nitrogen
ⓓ. a quaternary ammonium ion because nitrogen has four electron pairs
Correct Answer: a secondary amine because two carbon groups are attached to nitrogen
Explanation: The nitrogen is directly bonded to two carbon-containing groups: one methyl group and one ethyl group. It also retains one hydrogen and one lone pair. This gives the general pattern \(\mathrm{R_2NH}\), which is a secondary amine. Classification does not depend on whether all three substituents around nitrogen are different. A quaternary ammonium ion would require four groups bonded to nitrogen, a positive charge, and no lone pair.
15. Examine the classifications in the table.
| Row | Species | Carbon groups directly attached to nitrogen | Given classification |
|---|
| P | \(\mathrm{NH_3}\) | \(0\) | Primary amine |
| Q | \(\mathrm{C_2H_5NH_2}\) | \(1\) | Primary amine |
| R | \(\mathrm{(CH_3)_2NH}\) | \(2\) | Tertiary amine |
| S | \(\mathrm{(CH_3)_4N^+}\) | \(4\) | Tertiary amine |
The correctly classified row is:
ⓐ. Q only
ⓑ. P and Q only
ⓒ. Q and S only
ⓓ. R only
Correct Answer: Q only
Explanation: Ethylamine has one carbon group directly attached to nitrogen and therefore is a primary amine, so row Q is consistent. Ammonia has no carbon group attached to nitrogen and is the parent compound rather than a primary amine. Dimethylamine has two carbon groups attached to nitrogen and is secondary, not tertiary. Tetramethylammonium has four carbon groups and a positive charge, so it is a quaternary ammonium ion. The classification changes only when the number of carbon groups directly bonded to nitrogen changes.
16. Assertion: tert-Butylamine is a tertiary amine because the carbon atom bonded to \(\mathrm{NH_2}\) is tertiary.
Reason: The degree of an amine is determined by the number of carbon groups directly bonded to nitrogen.
ⓐ. Both Assertion and Reason are true, and Reason explains Assertion
ⓑ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Assertion is false, but Reason is true
Explanation: tert-Butylamine has the structure \(\mathrm{(CH_3)_3CNH_2}\). Although the carbon bonded to the amino group is a tertiary carbon, nitrogen itself is directly bonded to only one carbon group. The amine therefore has the pattern \(\mathrm{RNH_2}\) and is primary. The Reason states the proper basis for classifying amines. Carbon-atom degree and amine degree describe different structural features and must not be interchanged.
17. In \(\mathrm{C_6H_5NHCH_3}\), nitrogen is directly attached to one phenyl group, one methyl group, and one hydrogen atom. Its degree is:
ⓐ. primary because only one alkyl group is present
ⓑ. secondary because two carbon groups are attached to nitrogen
ⓒ. tertiary because an aromatic ring is counted as two substituents
ⓓ. quaternary because four different atoms are bonded around nitrogen
Correct Answer: secondary because two carbon groups are attached to nitrogen
Explanation: Both the phenyl group and the methyl group are carbon-containing groups directly bonded to nitrogen. Their different identities do not affect the degree of the amine. With two carbon groups and one hydrogen attached to nitrogen, the structure follows \(\mathrm{R_2NH}\). An aryl group counts as one group, not as several groups because it contains a ring. Whether the amine is simple or mixed is a separate description from whether it is primary, secondary, or tertiary.
18. Tetramethylammonium ion, \(\mathrm{(CH_3)_4N^+}\), differs from trimethylamine, \(\mathrm{(CH_3)_3N}\), because the ion:
ⓐ. has four carbon groups on positively charged nitrogen and no lone pair
ⓑ. has three carbon groups bonded to nitrogen and one available lone pair
ⓒ. contains one nitrogen-bonded hydrogen and is electrically neutral
ⓓ. has only two carbon groups directly attached to nitrogen
Correct Answer: has four carbon groups on positively charged nitrogen and no lone pair
Explanation: In tetramethylammonium ion, nitrogen forms four sigma bonds to four methyl groups. Formation of the fourth bond gives nitrogen a formal positive charge. Its valence electron pair is now involved in bonding, so no available lone pair remains. Trimethylamine has only three carbon groups, is neutral, and retains one lone pair. A quaternary ammonium ion is therefore not simply another name for a neutral tertiary amine.
19. When trimethylamine forms a bond with a methyl group to produce \(\mathrm{(CH_3)_4N^+}\), the nitrogen centre changes from:
ⓐ. two carbon groups and zero charge to three carbon groups and \(+1\) charge
ⓑ. three carbon groups and \(+1\) charge to four carbon groups and zero charge
ⓒ. three carbon groups and zero charge to four carbon groups and \(+1\) charge
ⓓ. four carbon groups and zero charge to three carbon groups and \(+1\) charge
Correct Answer: three carbon groups and zero charge to four carbon groups and \(+1\) charge
Explanation: Trimethylamine is a neutral tertiary amine with three methyl groups and one lone pair on nitrogen. Formation of an additional nitrogen–carbon bond uses that lone pair. The product contains four methyl groups directly bonded to nitrogen. A nitrogen atom with four such bonds carries a formal positive charge in the resulting ammonium ion. The structural change is therefore both an increase in substitution at nitrogen and a change in formal charge.
20. Evaluate the three statements below.
Statement I: Ammonia is not classified as a primary amine because no carbon group is bonded to nitrogen.
Statement II: A species of the form \(\mathrm{R_4N^+}\) is a quaternary ammonium ion.
Statement III: Every tertiary amine must contain three \(\mathrm{N-H}\) bonds.
Select the applicable combination.
ⓐ. I and III only
ⓑ. II and III only
ⓒ. I and II only
ⓓ. I, II and III
Correct Answer: I and II only
Explanation: Ammonia contains three nitrogen–hydrogen bonds but no nitrogen–carbon bond, so it is not a primary amine. A quaternary ammonium ion contains four carbon groups bonded to positively charged nitrogen, making Statement II valid. A tertiary amine has three carbon groups bonded to nitrogen and ordinarily has no \(\mathrm{N-H}\) bond. Statement III therefore reverses the actual hydrogen count. The prefixes primary, secondary, and tertiary track carbon-group substitution at nitrogen rather than the number of nitrogen–hydrogen bonds directly.
