Halogenoalkanes 1
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What is the general formula of a haloalkane
CnH2n+1X Where X is the halogen
What feature determines most of the physical properties of a halogenalkane
The polarity of the C-X bond.
Why is the C—X bond in a halogenalkane polar
The C—X bond is polar because halogens are more electronegative than carbon.
What happens to the C—X bond as you go down the halogen group (7)
It gets less polar
Define electronegativity
Electronegativity is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself
What are the main intermolecular forces of attraction in halogenalkane
Dipole-dipole attractions and Van Der Waals forces
Describe the solubility of halogenalkanes
• The polar C—X bond is not polar enough to make halogenalkanes soluble in water. • They are insoluble because they cannot form hydrogen bonds with the water.
Why are halogenalkanes used as dry-cleaning fluids
They remove oily stains. Oil is a mixture of hydrocarbons and halogenalkanes mix with hydrocarbons
What two things does the boiling point of a halogenalkane depend on
• the length of the carbon chain • the halogen present
How does the chain length affect the boiling point of a halogenalkane
The boiling point increases as the chain length increases. This is because the larger the molecule, the greater the number of electrons present and therefore the larger the Van Der Waals forces.
How does the halogen present affect the boiling point of a halogenalkane
The boiling point increases going down the halogen group. The size and the molecular mass of the halogen members increase as do the number of electrons present down the homologous series and therefore the Van Der Waals forces are stronger.
How do the boiling points of halogenalkanes compare to alkanes with similar length carbon chains
Halogenalkanes have higher boiling points because they have higher relative molecular masses and they are more polar.
Why does increased branching lower the boiling points of halogenalkanes
Because branched chains cannot pack together as closely so as unbranched chains and so the Van de Waals forces are not as effective.
What two factors affect how readily the C—X bond reacts
• The C—X bond polarity • The C—X bond enthalpy
Define nucleophile
A nucleophile is an electron pair donor
Describe how bond polarity affects the reactivity of the C—X bond in a halogenalkane
• The polarity of the C—X bond would suggest that the C—F bond is most reactive as it is most polar. • This is because fluorine is the most electronegative halogen. • This argument would suggest that the C—I bond is least reactive because it is least polar.
Describe the polarity of the C—X bond in a halogenalkane
• The halogens are more electronegative than carbon. • This means that the carbon bonded to the halogen has a partial positive charge- it is electron deficient. • This means that it can be attacked by reagents that are electron rich or have electron rich areas (nucleophiles)
Define bond enthalpy
Bond enthalpy is the energy required to break a particular covalent bond in one mole of molecule in a gaseous state.
Explain the trend in bond enthalpies for the C—X bond in a halogenalkane as you go down group 7
The bond enthalpies get weaker going down the group. This is because in the smallest atoms (fluorine) the shared electrons are strongly attracted to the fluorine nucleus. - As you go down the group, the shared electrons get further away from the nucleus and therefore the bonds get weaker.
Describe how the bond enthalpies affect the reactivity of the C—X bond
• The bond enthalpies would predict that iodo-compounds, with the weakest bonds are the most reactive and fluoro-compounds, with the strongest bonds are least reactive.
What is the conflict between trends in reactivity due to bond enthalpies and bond polarity and how have experiments helped with this.
• Bond polarity’s suggest that fluoro compounds are most reactive whereas bond enthalpies suggest iodo-compounds are most reactive • Experiments have confirmed that reactivity increases going down the group and this means that bond enthalpy is a more important factor than bond polarity.
List the features of a nucleophile
• A nucleophile is either a negatively charged ion or has an atom with a negative enthalpy charge. • A nucleophile has a lone (unshared) pair of electrons which it can use to form a covalent bond. • The lone pair is situated on an electronegative atom.
Summarise what happens in a nucleophile substitution reaction in a halgoenalkane
The nucleophile replaces the halogen atom in a halogenalkane.
What do the curly arrows in reaction mechanisms show
The movement of electron pairs in organic reactions.