Halogenoalkanes Practice Flashcards

This set of flashcards covers the introduction, naming, physical properties, reactivity, and major reactions (substitution and elimination) of halogenoalkanes, as well as their environmental impact.

Halogenoalkanes

Synthetic compounds consisting of an alkane skeleton with one or more halogen atoms (fluorine, chlorine, bromine, or iodine) in place of hydrogen atoms.

General formula (single halogen)

$C_nH_{2n+1}X$, where $X$ represents the halogen; often shortened to $R-X$.

Naming Convention: Prefixes

The prefixes fluoro-, chloro-, bromo-, and iodo- indicate which halogen is present in the compound.

Alphabetical Ordering in Naming

When a compound contains different halogens, they are listed in alphabetical order (e.g., 3-chloro-2-iodopentane).

Carbon-Halogen ($C-X$) Bond Polarity

The bond is polar, represented as $C^{\delta+}-X^{\delta-}$, because halogens are more electronegative than carbon.

Electronegativity of Carbon

The electronegativity value for carbon is $2.5$.

Electronegativity of Fluorine

The electronegativity value for fluorine is $4.0$.

Solubility of Halogenoalkanes

They are not soluble in water as the polar bonds are insufficient; they mix with hydrocarbons and can be used as dry-cleaning fluids.

Boiling Point Trends

Boiling point increases with increased chain length and as you go down the halogen group due to increased van der Waals forces.

Nucleophile

An electron pair donor that attacks and forms bonds with positively or partially positively charged carbon atoms.

Bond Enthalpy Trend ($C-X$)

Bond enthalpies decrease going down the group: $C-F$ ($467\,kJ\,mol^{-1}$), $C-Cl$ ($346\,kJ\,mol^{-1}$), $C-Br$ ($290\,kJ\,mol^{-1}$), and $C-I$ ($228\,kJ\,mol^{-1}$).

Reactivity Deciding Factor

Experiments confirm that bond enthalpy is a more important factor than bond polarity in determining the reactivity of halogenoalkanes.

Nucleophilic Substitution

A reaction mechanism where a nucleophile replaces the halogen atom in a halogenoalkane.

Curly Arrows

Symbols used in reaction mechanisms to show the movement of an electron pair, starting at a lone pair and moving toward an electron-deficient area.

Leaving Group

The halide ion ($X^-$) that breaks away from the halogenoalkane during a nucleophilic substitution reaction.

Hydrolysis of Halogenoalkanes

A reaction with aqueous sodium or potassium hydroxide that produces an alcohol ($ROH$).

Nitrile Formation

Produced when halogenoalkanes react with cyanide ions ($CN^-$); the resulting chain has one extra carbon atom.

Amine Formation

The product ($RNH_2$) of a reaction between a halogenoalkane and excess concentrated ammonia in ethanol under pressure.

Elimination Reaction

A reaction where a hydrogen halide is removed from a molecule, leaving a double bond and forming an alkene.

$OH^-$ acting as a Base

In hot, ethanolic conditions, the hydroxide ion removes an $H^+$ ion from the halogenoalkane rather than attacking the carbon atom.

Substitution vs. Elimination Conditions

Aqueous hydroxide at room temperature favours substitution; hot ethanolic hydroxide favours elimination.

Halogenoalkane Type Preference

Primary halogenoalkanes tend to react by substitution, while tertiary ones tend to react by elimination.

CFCs (Chlorofluorocarbons)

Halogenoalkanes containing chlorine and fluorine but no hydrogen; they decompose in the stratosphere to release chlorine atoms that destroy ozone ($O_3$).

HFCs (Hydrofluorocarbons)

Second-generation CFC replacements (e.g., $CHF_2CF_3$) that contain no chlorine and do not damage the ozone layer.