How does bond polarity change down Group 17 for halogenoalkanes?

Bond polarity decreases down the group. This is because the electronegativity of the halogen decreases from Fluorine to Iodine, reducing the difference in electronegativity between Carbon and the halogen.

Why is bond enthalpy more important than bond polarity in determining reactivity?

While polarity determines how much a nucleophile is attracted to the carbon, the reaction cannot proceed until the C-X bond breaks. The energy required to break the bond (enthalpy) is the limiting factor for the reaction rate.

Compare the rate of reaction between 1-chlorobutane and 1-iodobutane.

1-iodobutane reacts much faster than 1-chlorobutane. This is because the C-I bond is weaker (lower bond enthalpy) than the C-Cl bond, allowing it to break more easily during a substitution reaction.

What is a common misconception regarding the reactivity of fluoroalkanes?

A common error is assuming fluoroalkanes are the most reactive because the C-F bond is the most polar. In reality, they are the least reactive because the C-F bond is extremely strong and difficult to break.

What mistake is made if ethanol is omitted from the silver nitrate test?

Without ethanol, the halogenoalkane (which is non-polar) and the aqueous silver nitrate (which is polar) will form two separate layers. This prevents the reactants from mixing effectively, making the reaction extremely slow or non-existent.

Why might a student incorrectly predict that chloroalkanes react faster than iodoalkanes?

They might be focusing on electronegativity and bond polarity. Chlorine is more electronegative than Iodine, creating a larger $\delta+$ charge on the carbon, but this effect is outweighed by the much higher bond enthalpy of the C-Cl bond.

Define 'Bond Enthalpy' in the context of halogenoalkanes.

Bond enthalpy is the energy required to break one mole of the carbon-halogen bond. It determines the ease with which the halogen atom is displaced during a substitution reaction.

What is a nucleophile?

A nucleophile is an electron-pair donor. It is an atom or group of atoms that is attracted to an electron-deficient center (like the $\delta+$ carbon in a halogenoalkane) where it donates a pair of electrons to form a new covalent bond.

Hydrolysis is a chemical reaction in which a molecule is broken down by water. In the study of halogenoalkanes, it specifically refers to the substitution of a halogen atom by an -OH group from a water molecule.

Why does bond enthalpy decrease as the halogen atom gets larger?

As the atomic radius increases, the shared electrons in the C-X bond are further from the halogen nucleus. This increased distance results in a weaker electrostatic attraction and a longer, weaker bond.

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Chemistry

15. Halogen Compounds

Reactivity of Halogenoalkanes

Summary

The reactivity of halogenoalkanes is primarily determined by the strength of the carbon-halogen (C-X) bond rather than its polarity. While polarity creates a reactive center, the energy required to break the bond (bond enthalpy) is the rate-determining factor, leading to a clear trend where reactivity increases as you move down Group 17.

1. The Nature of the Carbon-Halogen Bond

Bond Polarity: Halogens are more electronegative than carbon, which creates a polar bond where the carbon atom carries a partial positive charge ( $\delta+$ ) and the halogen carries a partial negative charge ( $\delta-$ ).
Nucleophilic Attraction: The electron-deficient carbon atom ( $C^{\delta+}$ ) acts as an electrophilic center, attracting species with lone pairs of electrons known as nucleophiles.
Bond Enthalpy: This refers to the amount of energy required to break one mole of a specific bond in the gas phase. In halogenoalkanes, the C-X bond must break for a substitution reaction to occur.

Diagram showing the polar C-X bond with partial charges and the site of nucleophilic attack.

2. The Conflict: Polarity vs. Bond Enthalpy

3. Trend in Reactivity Down Group 17

Fluoroalkanes: These contain the strongest C-X bonds (highest bond enthalpy) and are effectively chemically inert under standard substitution conditions.
Chloroalkanes: These have moderate bond strength and react slowly with nucleophiles.
Bromoalkanes: These are more reactive than chloroalkanes due to a lower bond enthalpy.
Iodoalkanes: These are the most reactive halogenoalkanes because the C-I bond has the lowest bond enthalpy and is the easiest to break.

4. Experimental Verification: The Silver Nitrate Test

5. Key Distinctions: Rate vs. Polarity

6. Exam Strategy & Tips