ALKANES, ALKENES, AND ALCOHOLS Chapter 19

Homologous Groups:  Alkanes: hydrocarbon, only C and H  Alkenes: hydrocarbon, only C and H  Alcohols: Compound which contains an OH group  Homologous series: families of molecules with a general formula that have similar chemical properties and the physical properties change as the molecules get bigger.

As the molecules get bigger…  Properties change  This is mainly due to an increase in intermolecular attraction.  Boiling point increases, as larger molecules are more attracted to each other and therefore harder to pull apart.  They become less volatile. (Volatile substances turn to a vapour/evaporate easily)  Become more viscous, less runny, thicker.  They do not burn as easily: the smaller ones are more often used as fuels, easier to burn.

Increasing MP and BP with increasing number of carbon atoms

Increasing BP as size increases  Why does the BP increase as size increases?  The intermolecular attractions are stronger between larger molecules and more energy is needed to break these forces and separate the molecules.

The Alkanes  Names all end in an -ane  Saturated hydrocarbons: the carbon bonds are all single covalent bonds. They are all saturated with H atoms - the maximum number of H possible.  General formula: C n H 2n+2, CH 2 is added to each molecule.  Don’t forget how to name them and structural isomers.

Reactions of alkanes:  The smaller molecules are commonly used as fuels.  Other than burning, unreactive.  Methane combustion:  CH 4(g) +2O 2(g)  CO 2(g) + 2H 2 O (l)  Ethane combustion:  2C 2 H 6 + 7O 2(g)  4CO 2(g) + 6H 2 O (l)  Incomplete combustion: CO is produced when there is not enough O 2 available.

CO is dangerous!  CO binds more readily and irreversibly to haemoglobin, reducing the amount of O 2 that can be transported in the blood.  Cells do not receive enough O 2 and cannot carry out cellular respiration.

More reactions with alkanes:  Substitution reactions with halides, halogenation.  In UV light, a hydrogen can be replaced by a bromine or chlorine atom (substitution of one atom for another).  CH 4 + Br 2  CH 3 Br + HBr  We would see a colour change from brown to colourless.

The Alkenes:  Another family or homologous series.  Names all end in –ene  They are unsaturated: contain double bond(s).  General formula: C n H 2n  Alkenes burn well but aren’t used as fuels since they are more useful for other purposes – making polymers.  Are reactive in other ways due to the C=C, they undergo addition reactions. More reactive than alkanes.

Addition Reactions of alkenes  Chemical test for C=C:  Add brown bromine water.  The solution will decolourise, going from brown to colorless.  The double bond will ‘open’ to become a single bond and the 2 bromine atoms from Br 2 are added.  1,2-dibromoethane.

Alcohols  Homologous series “……ol”  Contain a functional group, -OH (hydroxyl) covalently bonded to a carbon chain.  General formula C n H 2n+1 OH  Most common alcohol: ethanol  What are the effects of ethanol:  Depresses some of the higher functions of the brain.  Depressive, releases inhibitions, affects concentration, slows reflexes, can cause social problems, dangerous to the liver if consumed in large quantities.

Production of Ethanol  Fermentation  Produced by the anaerobic respiration of sucrose by yeast.  Yeast is added to a sugar solution and left in a warm environment, o C (optimum is 35 o C) for several days in the absence of air.  Enzymes in the yeast catalyze the reaction and convert the sugar into ethanol and CO 2.  35 o C is optimum temperature for enzyme activity.  Enzymes will denature if the temperature is too high.

Overall equation includes 2 steps:  C 12 H 22 O 11 (aq) + H 2 O (l)  C 6 H 12 O 6 (aq)+ C 6 H 12 O 6 (aq)  Sucrose + water  glucose and fructose  Hydrolysis  C 6 H 12 O 6 (aq)  C 2 H 5 OH(aq) + 2CO 2 (g)  Fructose/glucose  ethanol + carbon dioxide  Sucrose is first broken down into glucose and fructose (isomers) by a hydrolysis reaction.  Yeast is killed when the mixture reaches about 15% alcohol, you cannot make pure alcohol by fermentation.

Distillation  Alcohol produced by fermentation can be purified by fractional distillation.  Alcohol boils at 78 o C, it condenses before water evaporates.  Condense the ethanol by cooling it down.  We can obtain 96% alcohol.

Production of Ethanol by hydration of ethene  Ethene + steam  ethanol  C 2 H 5 (g) + H 2 O (g)  CH 3 CH 2 OH (g)  Only a small proportion of ethene reacts at a time.  The ethanol produced is condensed as a liquid and the unreacted ethene is recycled back through the process  This is a hydration reaction – adding water.  The ethene is obtained from crude oil.  Phosphoric acid is the catalyst.  Pressure = 60 atmospheres.

Reactions with ethanol  Burning: alternative fuel for cars  Dehydration: removal of water to form ethene  Heat and a catalyst are required