Nomenclature, Nature of C-X Bond — Revision Notes

⚡ 30-Second Revision

Haloalkanes — Alkanes with H replaced by X (F, Cl, Br, I).
Classification — $1^\circ$ (C-X bonded to 1 R group), $2^\circ$ (2 R groups), $3^\circ$ (3 R groups).
IUPAC Naming — 'Haloalkane' system. Longest chain, lowest locants for substituents, alphabetical order (e.g., 2-bromopropane).
Common Naming — 'Alkyl halide' system (e.g., isopropyl bromide).
C-X Bond Polarity — Halogen is $\delta^-$, Carbon is $\delta^+$. Electrophilic carbon.
Bond Length Trend — C-F < C-Cl < C-Br < C-I (increases down group).
Bond Strength Trend — C-F > C-Cl > C-Br > C-I (decreases down group).
Dipole Moment Trend — $CH_3Cl > CH_3F > CH_3Br > CH_3I$ (C-Cl anomaly).
Leaving Group Ability — $I^- > Br^- > Cl^- > F^-$ (weaker bond = better leaving group).

2-Minute Revision

Haloalkanes are alkane derivatives with a halogen atom. They are classified as primary ($1^\circ$), secondary ($2^\circ$), or tertiary ($3^\circ$) based on the number of alkyl groups attached to the carbon bearing the halogen.

This classification is crucial for understanding reactivity. Naming follows two systems: common names (alkyl halide, e.g., methyl chloride) and IUPAC names (haloalkane, e.g., chloromethane). IUPAC rules require identifying the longest carbon chain, numbering it to give the lowest possible locants to substituents, and listing them alphabetically.

The C-X bond is highly polar due to the halogen's higher electronegativity, making the carbon atom partially positive ($\delta^+$) and susceptible to nucleophilic attack. Key trends for the C-X bond are: bond length increases down the halogen group (C-F to C-I), while bond strength decreases.

The C-F bond is the strongest, and C-I is the weakest. An important exception is the dipole moment: $CH_3Cl$ has a higher dipole moment than $CH_3F$ due to the longer C-Cl bond length compensating for slightly less charge separation.

These bond properties directly influence the leaving group ability, with iodide being the best leaving group.

5-Minute Revision

Haloalkanes, or alkyl halides, are fundamental organic compounds where a halogen (F, Cl, Br, I) replaces a hydrogen in an alkane. Their structure is often denoted as $R-X$. Classification is based on the carbon atom directly bonded to the halogen: if it's attached to one alkyl group, it's primary ($1^\circ$); two, secondary ($2^\circ$); three, tertiary ($3^\circ$). Methyl halides ($CH_3X$) are considered primary. This classification is vital for predicting reaction mechanisms.

Nomenclature is critical. Common names use the alkyl group name followed by the halide (e.g., ethyl bromide for $CH_3CH_2Br$). IUPAC names are systematic: the halogen is treated as a 'halo-' substituent on the parent alkane chain.

Rules include finding the longest carbon chain, numbering it to give the lowest possible locants to all substituents (halogens and alkyl groups), and listing them alphabetically. For example, $CH_3CH(Cl)CH_2CH_3$ is 2-chlorobutane.

Dihaloalkanes can be geminal (halogens on the same carbon, e.g., 1,1-dichloroethane) or vicinal (halogens on adjacent carbons, e.g., 1,2-dichloroethane).

The Nature of the C-X Bond is central to haloalkane chemistry. Halogens are more electronegative than carbon, making the C-X bond polar. The carbon atom acquires a partial positive charge ($\delta^+$), making it an electrophilic center, vulnerable to nucleophilic attack. The halogen acquires a partial negative charge ($\delta^-$). Key trends are:

1
Bond Length — Increases down the group (C-F < C-Cl < C-Br < C-I) due to increasing atomic size.
2
Bond Strength (Dissociation Enthalpy) — Decreases down the group (C-F > C-Cl > C-Br > C-I) because longer bonds are weaker. C-F is the strongest bond.
3
Dipole Moment — While F is most electronegative, $CH_3Cl$ has a slightly higher dipole moment than $CH_3F$. This is because the longer C-Cl bond length compensates for the slightly smaller charge separation compared to C-F. The general trend is $CH_3Cl > CH_3F > CH_3Br > CH_3I$.
4
Leaving Group Ability — Weaker C-X bonds mean better leaving groups. Thus, $I^-$ is the best leaving group, followed by $Br^-$, $Cl^-$, and $F^-$. This directly impacts reaction rates.

Worked Example: Name $CH_3CH(CH_3)CH(Br)CH_2CH_3$ and classify it.

1
Longest chain — 5 carbons (pentane).
2
Numbering — From left: methyl at C2, bromo at C3 (2,3). From right: bromo at C3, methyl at C4 (3,4). Lowest locants are (2,3). So, number from left.
3
Substituents — Bromo at C3, methyl at C2. Alphabetical order: bromo before methyl.
4
IUPAC Name — 3-Bromo-2-methylpentane.
5
Classification — The carbon at C3 (bonded to Br) is attached to two other carbon atoms (C2 and C4). Thus, it is a secondary ($2^\circ$) haloalkane.

Prelims Revision Notes

Nomenclature of Haloalkanes

Definition — Alkanes where one or more H atoms are replaced by X (F, Cl, Br, I).
General Formula — $R-X$ for monohaloalkanes.
Classification — Based on the carbon atom bearing the halogen ($C_X$).

* **Primary ($1^\circ$)**: $C_X$ bonded to one alkyl group (e.g., $CH_3CH_2Cl$). Methyl halides ($CH_3X$) are also $1^\circ$. * **Secondary ($2^\circ$)**: $C_X$ bonded to two alkyl groups (e.g., $CH_3CH(Br)CH_3$). * **Tertiary ($3^\circ$)**: $C_X$ bonded to three alkyl groups (e.g., $(CH_3)_3CCl$).

Common Names (Alkyl Halide System)

Name alkyl group + halide (e.g., methyl chloride, isopropyl bromide, tert-butyl iodide). Used for simpler compounds.

IUPAC Names (Haloalkane System)

Halogen treated as a 'halo-' prefix (fluoro-, chloro-, bromo-, iodo-). Rules: 1. Identify longest continuous carbon chain (parent alkane). 2. Number chain from end giving lowest locants to substituents (halogens, alkyl groups).

3. If tie in locants, prioritize alphabetical order of substituents. 4. List substituents alphabetically with their locants (di-, tri- prefixes ignored for alphabetization). 5. Examples: $CH_3CH_2Cl$ (Chloroethane), $CH_3CH(Br)CH_3$ (2-Bromopropane), $CH_3CH(Cl)CH(CH_3)CH_3$ (2-Chloro-3-methylbutane).

* Dihaloalkanes: * Geminal: Both halogens on same carbon (e.g., 1,1-dichloroethane). * Vicinal: Halogens on adjacent carbons (e.g., 1,2-dichloroethane).

Nature of C-X Bond

Polarity — C-X bond is polar due to electronegativity difference (X > C).

* Carbon is $\delta^+$, Halogen is $\delta^-$. * Makes carbon electrophilic, susceptible to nucleophilic attack.

Electronegativity Order — F > Cl > Br > I.
Bond Length Trend — Increases down the group (C-F < C-Cl < C-Br < C-I).

* C-F: $\approx 139\,\text{pm}$ * C-Cl: $\approx 178\,\text{pm}$ * C-Br: $\approx 193\,\text{pm}$ * C-I: $\approx 214\,\text{pm}$

Bond Strength (Bond Dissociation Enthalpy) Trend — Decreases down the group (C-F > C-Cl > C-Br > C-I).

* C-F: $\approx 452\,\text{kJ/mol}$ (Strongest) * C-Cl: $\approx 351\,\text{kJ/mol}$ * C-Br: $\approx 293\,\text{kJ/mol}$ * C-I: $\approx 234\,\text{kJ/mol}$ (Weakest)

Dipole Moment Trend — $CH_3Cl > CH_3F > CH_3Br > CH_3I$.

* Anomaly: $CH_3Cl$ has higher dipole moment than $CH_3F$ due to longer bond length compensating for slightly less charge separation.

Leaving Group Ability — Directly related to bond strength. Weaker bond = better leaving group.

* $I^- > Br^- > Cl^- > F^-$ (Iodide is the best leaving group).

Vyyuha Quick Recall

For C-X bond properties: Longer Bonds are Weaker. Think Length, Breaking energy, Weakness. As you go down the halogen group (F to I), atomic size increases, so bond Length increases, bond Breaking energy (strength) decreases, and thus the bond becomes Weaker. For dipole moment, remember the 'Cl-F Flip': $CH_3Cl$ has a higher dipole moment than $CH_3F$.