Alkanes and alkenes

What are alkanes?

Hydrocarbons that contain only carbon - carbon or carbon - hydrogen single bonds

What is the general formula for alkanes?

CnH2n+2

Describe the melting and boiling points of alkanes and alkenes in general and down the series

• Gradually increase down the homologous series

  • Forces of attraction between molecules increase as their size increases

• Gases at r.t.p.

• Simple molecular substances with weak forces of attraction between molecules

Describe the viscosity of alkanes and alkenes

• Alkanes become more viscous as their molecular size increases

  • Stronger forces of attraction between alkane molecules, restricting flow

• Longer chains lead to molecules getting entangled

Describe the solubility of alkanes and alkenes

• Insoluble in water but soluble in most organic solvents

• Liquid alkanes are often used as solvents for other compounds

What are the conditions for the combustion of alkanes and alkenes? (+O2)

Excess oxygen for complete combustion and limited oxygen for incomplete combustion

What is produced during the combustion of alkanes and alkenes?

• CO2 + H2O (excess O2)

• CO + H2O (limited O2)

Why is combustion of alkanes harder down the series?

Mr increases → flammability decreases → incomplete combustion

What are the conditions for substitution of alkanes?

• Presence of UV light

  • Due to alkanes’ unreactivity → requires very high energy

• Limited halogen (mono-sub.)

• Excess halogen (multi-sub.)

What is the substitution of alkanes?

One or more hydrogen atoms in an alkane are replaced by other atoms like halogen (e.g. Cl and Br) atoms

Diagrams of alkanes

What are alkenes?

Hydrocarbons that contain C = C double bonds

*C = C functional group

What is the general formula for alkenes?

CnH2n

How is the combustion of alkenes as compared to alkanes?

Alkenes → sootier flame

Alkanes → less sooty flame

Are alkenes likely to undergo complete or incomplete combustion?

Incomplete

What are the conditions for the hydrogenation of alkenes? (addition of hydrogen)

• 150ºc

• Presence of a nickel catalyst

Compare the reactivity of alkenes and alkanes

Alkenes → reactive

Alkanes → generally unreactive

What are the conditions for bromination of alkenes? (addition of bromine)

• Room temperature

• Absence of UV light

What is the color change of bromination of alkenes?

Reddish-brown → colourless

What are the conditions for the hydration of alkenes? (addition of steam)

• 300ºc

• 60atm

• Presence of H3PO4 [phosphoric (V) acid] as a catalyst

What is the product of the hydration of alkenes?

Alcohol

What are the conditions of addition polymerization of alkenes?

• Pressure

• Presence of catalyst

What are the conditions for the production of alkenes via cracking of alkanes? (decomposition)

• 500ºc to 700ºc

• 1atm

• Presence of Al2O3 or SiO2 as a catalyst

Describe the electrical conductivity of alkenes and alkanes

Do not conduct electricity in any state

How do alkenes and alkanes react differently with bromine?

Alkanes → bromine remains orange

Alkenes → orange bromine decolourises

What are the differences between saturated and unsaturated compounds?

Saturated → C - C single covalent bonds with all carbon atoms bonded to 4 other atoms

Unsaturated → C = C double covalent bonds with more atoms added across the carbon

What is the test for unsaturation?

1. Add aqueous bromine to each sample in the absence of UV light

2. Br (aq) remains brown when shaken with saturated hydrocarbons (alkane)

   • Alkanes undergo substitution with halogen only in the presence of UV light

3. Br (aq) decolourises when shaken with unsaturated hydrocarbons (alkenes)

   • Alkenes undergo halogenation (addition) spontaneously even when light is absent

Do saturated or unsaturated fats have higher molecular mass and why?

• Saturated fat has relatively higher molecular mass / size

• Hydrogen atoms are added to each C = C bond

• The greater the level of saturation, the higher the melting point

Do saturated or unsaturated fats pack more closely to one another and why?

• Saturated fats are more linear in shape, so they clot our arteries more effectively

• Unsaturated fats have less linear and more irregular shapes, so they cannot clot our arteries effectively

What are the states of saturated and unsaturated fats at r.t.p. and pressure

Saturated → solids → strong IFA, can stack well

Unsaturated → liquids → weak IFA, cannot stack well

What is glucose from plants converted to?

Ethanol and CO2 using yeast in aqueous solution at temperatures below 30ºc

How can margarine be manufactured from vegetable oil?