Halogenoalkanes
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What is the general formula of a halogenoalkane?
CnH2n+1X (where x = halogen)
What do the prefixes (iodo-, fluoro-, bromo- and chloro-) tell about the halogenoalkane?
Dictate which halogen are present
Why is the C-X bond in halogenoalkanes polar?
The C-X bond is polar because halogens are more electronegative than carbon
How does bond polarity change as you go down Group 7 in halogenoalkanes?
As you go down Group 7, the electronegativity of the halogens decreases, making the C-X bond less polar
Why are halogenoalkanes not soluble in water?
The C-X bond is not polar enough to form hydrogen bonds with water molecules
What are the main intermolecular forces present in halogenoalkanes?
The main intermolecular forces are dipole - dipole interactions and van der Waals forces
What two structural features increase the boiling point of halogenoalkanes?
Increased chain length and bigger halogen atoms (down the group)
How does the chain length affect the boiling point of halogenoalkanes? (length inc, more e-, van der waals)
As chain length increases, there are more electrons present, leading to stronger van der Waals forces increasing boiling point
How does the halogen in a halogenoalkane affect its boiling point? (down, size up, van der waals, more electrons)
As you down Group 7, halogen atoms increase in size resulting in stronger van der Waals forces due to the increased presence of electrons
What effect does branching have on the boiling point of halogenoalkanes?
Increased branching lowers the boiling point because molecules pack less closely, leading to weaker van der Waals
Why do halogenoalkanes have higher boiling points than alkanes of similar chain length?
Halogenoalkanes have higher relative molecular masses and are more polar, leading to stronger intermolecular forces compared to alkanes
When halogenoalkanes react, which bond typically breaks?
The C-X bond (carbon - halogen bond) is the one that breaks during reactions
What are the two factors that determine the reactivity of the C-X bond?
• C-X bond polarity • C-X bond enthalpy
What type of reagents attack the electron-deficient carbon in halogenoalkanes?
Nucleophiles, which are electron pair donors
According to bond polarity, which C-X bond should be the most reactive, and why?
C-F should be the most reactive, because is the most polar making the carbon have the greatest slight positive charge making ti easier for the nucleophile to attack
According to bond polarity, which C-X bond should be the most reactive?
The C-I should be the least reactive, because it is the least polar
What does bond enthalpy predict about reactivity?
Weaker bonds will react more easily, so iodo- compounds are most reactive and fluoro- compounds least reactive
Why does the C-X bond get weaker down the group?
Down the group, halogens get larger, the shared electrons become further from the nucleus, resulting in a weaker attraction and a weaker bond
Based on bond enthalpies, which halogenoalkane is the most/least reactive?
Fluoro compounds least reactive and iodo compounds most reactive
Which factor is more important in determining halogenoalkane reactivity, and why?
Bond enthalpy is more important than bond polarity. This is because experiments have shown reactivity increases as you go down the group, following the trend of decreasing bond strength