Alkanes Flashcards
What is the general formula of an alkane?
CnH2n+2
What does the term ‘hydrocarbon’ mean?
Containing only carbon and hydrogen atoms.
Why are alkanes described as ‘saturated’?
They are saturated as alkanes only contain single bonds.
How do cycloalkanes differ from alkanes?
Cycloalkanes take a cyclic form and has the general formula CnH2n.
What is the shape around every carbon atoms in alkanes?
It is a tetrahedral shape with 109.5 degrees bond angle.
Why does a longer non-branched alkane have a higher boiling point than a shorter one?
A longer non-branched alkane has a higher boiling point because:
> There will be more surface contact.
> This means more induced dipole-dipole forces can form.
> Hence, more energy is needed to overcome these induced dipole- dipole forces.
Why do branched alkanes have a lower boiling point than unbranched alkanes?
Branched alkanes have a lower boiling point because:
> They cannot pack closely together.
> Less induced dipole-dipole forces can form as there is less surface contact.
> As there is less induced dipole dipole forces, less energy is needed to overcome these forces.
What is bond fission?
The breaking of a covalent bond.
What is the difference between homolytic and heterolytic bond fission?
In homolytic bond fission, the bond breaks so the pair of electrons in the bond is shared equally to the two bonding atoms to form 2 uncharged radicals. In heterolytic fission, the bond breaks so the pair of electrons only goes to one bonding atom, to form a cation and anion.
What are the three stages of the free radical substitution mechanism?
Initiation- Free radicals are produced via homolytic bond fission, initiated by UV light. Photochemical reaction.
Propagation- Radicals enter a chain reaction due to high reactivity.
Termination- 2 radicals react to form a stable compound. Here the chain reaction stops.
What bond fission occurs in the initiation stage of a free radical substitution mechanism? What conditions are needed?
Homolytic bond fission occurs to form 2 radicals, with the conditions of UV light.
Halogens and alkanes react together to form haloalkanes. What mechanism does this occur by?
Free radical substitution mechanism.
Show the free radical substitution mechanism of methyl reacting with chlorine.
Cl-Cl > Cl* + Cl* (UV light)
…..
CH4 + Cl* > HCl + CH3*
CH3* + Cl2 > CH3Cl + Cl*
……
Cl* + Cl* > Cl2
Cl* + CH3* > CH3Cl
CH3* + CH3* > C2H6
What is the problem with free radical substitution mechanisms?
Multiple substitutions can occur, so when the reaction finishes in unknown. As there are multiple substitutions, a mixture of products can be formed, as substitutions can occur on more than one carbon in a compound. The substitution the carbon occurs on is unknown, so we get structural isomers of a compound.
