ORGANIC MOLECULES1.STRUCTURE,PHYSICAL

PROPERTY RELATIONSHIPS.

AND

2.REACTIONS OF THE HYDROCARBONS

STRUCTURE

• Chain length• Branched chains• Number and type of functional group• Intermolecular forces(Hydrogen bonding and

Van der Waals forces)

PHYSICAL PROPERTIES

• Melting point• Boiling point• Vapour pressure• Viscosity

PHYSICAL PROPERTIES

• Melting, boiling points and viscosity(ability to flow) are dependent on chain length.(chain length affects molecular mass)

• These properties(characteristics) are also dependent on whether they are straight chains, branched chains or cyclic compounds.

• Organic compounds are generally held together by weak intermolecular forces(Van Der Waals forces)

PHYSICAL PROPERTIES CONT.

• In general, the longer the chain length , the bigger the molecular mass ,the higher the melting and boiling points(WHY?).

• Branches in the chain tend to decrease the melting and boiling points(WHY?)

• Alcohols and carboxylic acids contain the -OH groups and hydrogen bonds exists between their molecules-melting and boiling points and viscosity increase(can you think of a reason?)

PHYSICAL PROPERTIES CONT.

• Solubility of alkanes in water is also affected by their chain length(the longer the chain,the more insoluble)

• Carbonyl group(-C=O) in ketones and aldehydes is very polar-these compounds are very soluble in water.

• BOILING POINT VS VAPOUR PRESSURE!!(what relationship exists here?)

Vapour pressure vs boiling point

• Vapour pressure is the pressure exerted by a vapour(this is an indication of the ease of evaporation of a liquid).

• A substance with a high vapour pressure is referred to as volatile.

• Substances with low boiling points will have higher vapour pressures(it is easier to form the vapour-it is easy to boil or evaporate)

• Substances with high boiling points will have lower vapour pressures(it is difficult to form the vapour it is difficult to boil or evaporate)

REACTIONS OF THE HYDROCARBONS

• OXIDATION(COMBUSTION)ALKANES,ALKENES AND ALKYNES CAN UNDERGO OXIDATION REACTIONS IN AIR.(THESE REACTIONS ARE VERY EXOTHERMIC-MOST COMPOUNDS ARE USED AS FUELS)PRODUCTS ARE CARBON DIOXIDE AND WATERC3H8 + 5O2 3CO2 + 4H2O

REACTIONS CONT.

• ADDITIONTHIS IS USED IN POLYMERIZATION.TWO MOLECULES JOIN TOGETHER TO FORM A SINGLE NEW MOLECULE.UNSATURATED COMPOUNDS UNDERGO ADDITION REACTIONS IN WHICH THE DOUBLE OR TRIPLE BOND BREAKS OPEN ALLOWING FOR ADDITION OF EXTRA ATOMS.DEPENDING ON COMPOUNDS ADDED,REACTIONS ARE GIVEN SPECIFIC NAMES

REACTIONS CONT.• HYDROHALOGENATION(ADDITION OF HX)

CH2=CH2 + HCl CH3CH2Cl• HALOGENATION(ADDITION OF A HALOGEN)

CH2=CH2 + Br2 CH2BrCH2Br

• HYDRATION/HYDROLYSIS(ADDITION OF H2O)

CH2=CH2 + H2O CH3CH2OH

• HYDROGENATION(ADDITION OF H2)

CH2=CH2 + H2 CH3CH3

REACTIONS CONT.

NB! IN HYDROHALOGENATION OR HYDRATION/HYDROLYSIS,A SPECIAL RULE(MARKOVNIKOV`s RULE) MUST BE APPLIED-THE H-atom JOINS TO THE C-atom OF THE DOUBLE BOND THAT HAS THE MOST H-atoms ON IT.

REACTIONS CONT.

• ELIMINATION REACTIONSTHESE ARE THE REVERSE OF ADDITIONSOMETHING IS REMOVED FROM A SUBSTANCE AND THUS ONE COMPOUND FORMS TWO .DEPENDING ON WHAT IS ELIMINATED,THE REACTION IS GIVEN A SPECIAL NAME.

REACTIONS CONT.

• DEHYDROHALOGENATION(REMOVAL OF HX)CH2ClCH2Cl CH2=CHCl + HCl

• DEHYDRATION(REMOVAL OF H2O FROM AN ALCOHOL)CH3CH2OH CH2=CH2 + H2O

• DEHYDROGENATION(REMOVAL OF HYDROGEN)CH3CH2CH3 CH3CH=CH2 + H2

REACTIONS CONT.

• SUBSTITUTION REACTIONS(THIS INVOLVES THE REPLACEMENT OF AN ATOM OR A GROUP OF ATOMS BY ANOTHER ATOM OR GROUP OF ATOMS)

REACTIONS OF HX WITH OHCH3CH2OH + HCl CH3CH2Cl + H2O

REACTIONS OF OH WITH HXCH3CH2Cl + KOH CH3CH2OH + KCl

REACTIONS CONT.

HALOGENATIONCH3CH3 + Br2 CH2BrCH2Br + H2

• ESTERIFICATIONmethanol + ethanoic acid conc.H2SO4 methyl ethanoate +H2O

ethanol + methanoic acid conc H2SO4 ethyl methanoate +H2O

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