Q.Complete the following acid-base reactions and name the products:

The key idea is that amines act as bases by donating their lone pair to a proton ($H^+$), forming an ammonium salt.

Step 1: In both reactions, the amine (a Lewis base) accepts a proton from HCl (a Lewis acid).

Step 2: The lone pair on nitrogen forms a coordinate bond with $H^+$, producing a positively charged ammonium ion. The chloride ion ($Cl^-$) remains as the counterion.

Step 3: The products are alkylammonium chlorides — water-soluble ionic salts.

1. $CH_3CH_2CH_2NH_2 + HCl \rightarrow CH_3CH_2CH_2\overset{+}{N}H_3Cl^-$ Product: Propylammonium chloride (or n-propylammonium chloride).
2. $(C_2H_5)_3N + HCl \rightarrow (C_2H_5)_3\overset{+}{N}HCl^-$ Product: Triethylammonium chloride.
   ✓Final answer

   The products are propylammonium chloride and triethylammonium chloride, respectively.

The Concept: Why Amines Act as Bases

Amines are organic derivatives of ammonia ($NH_3$). The nitrogen atom has a lone pair of electrons that is available for sharing with a proton ($H^+$). When an amine meets a strong acid like hydrochloric acid ($HCl$), the acid donates its proton to the amine's lone pair. This is a Lewis acid-base reaction (the amine is the Lewis base, $H^+$ is the Lewis acid) and also a Brønsted-Lowry acid-base reaction (the amine accepts a proton).

The result is an ammonium salt — the nitrogen now has a positive charge and is bonded to four groups (three alkyl/aryl groups plus the new hydrogen). The chloride ion from HCl becomes the counterion.

Step-by-Step Solution

1. Reaction (i): $CH_3CH_2CH_2NH_2 + HCl \rightarrow$

Identify the base: $CH_3CH_2CH_2NH_2$ is propylamine (a primary amine). The nitrogen has a lone pair.

Identify the acid: $HCl$ is a strong acid, fully dissociating into $H^+$ and $Cl^-$ in aqueous medium.

The proton transfer: The lone pair on nitrogen attacks the proton from HCl. The nitrogen becomes positively charged (now tetravalent) and gains one more hydrogen.

The reaction is:

Naming the product: The cation is named by replacing the "-amine" suffix with "-ammonium" and adding the name of the alkyl group. So $CH_3CH_2CH_2NH_3^+$ is propylammonium ion. The salt is propylammonium chloride.

2. Reaction (ii): $(C_2H_5)_3N + HCl \rightarrow$

Identify the base: $(C_2H_5)_3N$ is triethylamine (a tertiary amine). All three hydrogens of ammonia are replaced by ethyl groups. The nitrogen still has a lone pair.

The proton transfer: Same mechanism — the lone pair accepts $H^+$ from HCl.

Naming the product: The cation is triethylammonium ion (three ethyl groups + one H on nitrogen). The salt is triethylammonium chloride.

Summary Table

Method: Acid–Base Neutralisation of Amines

This is a Bronsted–Lowry acid–base reaction. The amine acts as a base (proton acceptor) and HCl acts as the acid (proton donor). The product is an ammonium salt.

Steps

1. Identify the basic site — the lone pair on the nitrogen atom of the amine.
2. Proton transfer — the nitrogen donates its lone pair to the proton ($H^+$) from HCl, forming a coordinate bond.
3. Form the salt — the resulting positively charged ammonium ion pairs with the chloride ion ($Cl^-$).

(i) $CH_3CH_2CH_2NH_2 + HCl \rightarrow$

• Amine: propylamine (primary amine)
• Product: propylammonium chloride

Product name: Propylammonium chloride

(ii) $(C_2H_5)_3N + HCl \rightarrow$

• Amine: triethylamine (tertiary amine)
• Product: triethylammonium chloride

Product name: Triethylammonium chloride

Key Exam Point

In such reactions, the amine is the base and HCl is the acid. The product is always an ammonium salt — named by replacing “amine” with “ammonium” and adding the anion name (chloride, sulfate, etc.).

Here are the common mistakes students make with acid-base reactions of amines, specifically for the two reactions you listed, along with how to avoid each.

Mistake 1: Forgetting that Amines are Bases

Students often treat amines as neutral compounds that simply "react" with acids, failing to recognize the proton transfer (Brønsted-Lowry acid-base) mechanism.

• The Error: Writing the product as a simple mixture (e.g., $CH_3CH_2CH_2NH_2 + HCl \rightarrow CH_3CH_2CH_2NH_2 + HCl$) or incorrectly breaking the C-N bond.
• How to Avoid: Remember the lone pair on the nitrogen atom. It acts as a base, accepting a proton ($H^+$) from the acid. The reaction is:

The product is always an **ammonium salt** (an alkylammonium cation paired with a halide ion).

Mistake 2: Incorrectly Naming the Salt Product

Students often name the product as "amine hydrochloride" but get the specific alkyl group wrong, or they forget the "chloride" part entirely.

• The Error: For reaction (i), writing "propylammonium chloride" but missing the "propyl" prefix, or writing "propaneammonium chloride" (incorrect IUPAC style). For reaction (ii), writing "triethylammonium chloride" as "triethylamine chloride" (missing the "-ium" suffix).
• How to Avoid: Follow this naming rule:
  • Step 1: Replace the "-amine" suffix of the parent amine with "-ammonium".
  • Step 2: Add the name of the acid's anion (e.g., chloride, sulfate, nitrate).
  • Examples:
    • $CH_3CH_2CH_2NH_2$ (propylamine) $\rightarrow$ Propylammonium chloride
    • $(C_2H_5)_3N$ (triethylamine) $\rightarrow$ Triethylammonium chloride

Mistake 3: Forgetting the Charge on the Product

Students write the product as a neutral molecule (e.g., $CH_3CH_2CH_2NH_3$) instead of an ionic salt.

• The Error: Writing $CH_3CH_2CH_2NH_3$ (neutral) instead of $CH_3CH_2CH_2NH_3^+ Cl^-$ (ionic).
• How to Avoid: Always check the octet rule and formal charge. After accepting a proton, nitrogen has four bonds and a positive formal charge. The acid's conjugate base (e.g., $Cl^-$) is a separate, negatively charged ion. The product is an ionic compound — write it as separate ions or as a salt formula (e.g., $[CH_3CH_2CH_2NH_3]Cl$).

Mistake 4: Confusing Primary, Secondary, and Tertiary Amines

Students think tertiary amines (like triethylamine) cannot react because they have no N-H bond.

• The Error: Claiming $(C_2H_5)_3N$ does not react with $HCl$ because "it has no hydrogen to donate."
• How to Avoid: Remember that basicity depends on the lone pair, not on N-H bonds. Tertiary amines have a lone pair on nitrogen and are actually stronger bases than primary amines in the gas phase (though in water, solvation effects make secondary amines slightly stronger). They readily accept a proton to form a trialkylammonium salt — note this is not a “quaternary ammonium” salt: quaternary means four C–N bonds (like $R_4N^+$), whereas $(C_2H_5)_3NH^+$ still has an N–H bond:

Mistake 5: Writing the Wrong Stoichiometry

Students use more than one mole of acid per mole of amine, even for simple monoamines.

• The Error: Writing $CH_3CH_2CH_2NH_2 + 2HCl \rightarrow CH_3CH_2CH_2NH_3Cl_2$ (a dihydrochloride).
• How to Avoid: Each basic nitrogen atom accepts one proton. For a monoamine (one nitrogen), the reaction is 1:1 with a monoprotic acid like $HCl$. Only diamines (e.g., $H_2N-CH_2-CH_2-NH_2$) require two moles of acid.

Summary Table: Correct Answers

Final Tip: Always draw the lone pair on the nitrogen before starting. Then, show the arrow from the lone pair to the $H^+$ of the acid. This visual step prevents all the mistakes above.