8/9/2019 As Topic 5 Notes - Alkanes

1/5

Topic 5a – Introduction to Organic ChemistryRevision Notes

1) Formulae

Be able to recognise and use the diferent ways o showing organic compounds:

• Molecular ormula is the actual number o atoms o each element in amolecule e.g. C 2H6O or ethanol

• !mpirical ormula is the simplest whole number ratio o the atoms oeach element in a molecule e.g. CH 2 or ethene ( rom molecular ormulaC2H ÷ 2!

• "eneral ormula is the simplest algebraic ormula or a member o ahomologous series e.g. C nH2n"2 or al#anes

• #tructural ormula is the minimum detail that shows the arrangement othe atoms in a molecule e.g. CH $CH 2OH or ethanol

• $isplayed ormula shows the relati%e positioning o atoms and the bondsbetween them e.g. or ethanol:

&ll bonds should be shown. $o not put 'OH or the alcohol group

%) Functional groups and naming organic compounds

Be able to recognise and use the ollowing terms:

• & homologous series is a series o organic compounds ha%ing the sameunctional group with successi%e members difering by CH 2

• &l#anes al#enes alcohols and halogenoal#anes are all homologous series• & unctional group is a group o atoms responsible or the characteristic

reactions o a compound e.g. C)C or al#enes and 'OH or alcohols

*he rules or naming organic compounds were de%ised by +,-&C (+nternational,nion o -ure and &pplied Chemistry!. *hey are as ollows.

! *he unctional group gi%es the ending o the name e.g. 'ol or analcohol

2! *he number o carbons gi%es the /rst part o the name e.g. prop0 orpropan0 or $ carbons

$! 1umber the carbon chain to gi%e the unctional group carbon thelowest number

! &ny side chains (branches! or halogens go at the ront o the name withcommas between numbers and dashes between numbers and wordse.g. 2 20dimethylhe ane

3! 4ith more than side chain or halogen use alphabetical order e.g. 0bromo020methylbutane

8/9/2019 As Topic 5 Notes - Alkanes

2/5

') #tructural isomers

• 5tructural isomers ha%e the same molecular ormula but diferentstructural ormulae

• *here are $ types o structural isomers: chain position and unctionalgroup

• (osition isomers difer in the location o the unctional group e.g.

0bromobutane 20bromobutane

• Chain isomers ha%e diferent arrangements o the carbon chain e.g.

-entane 20methylbutane

• & molecular ormula can be common to compounds rom diferent amilies. *hese are unctional group isomers e.g.

o C$H6O can be either propanal CH $CH 2CHO or propanone CH $COCH $o C$H6 can be either propene or cyclopropaneo C$H6O 2 can be propanoic acid methyl ethanoate or ethyl methanoateo C2H6O can be either ethanol or metho ymethane CH $OCH $ (an ether!

• For C !M1* the only pair o unctional group isomers needed isal+enes and cyclic al+anes ,-oth o .hich have general ormulaC n %n)

8/9/2019 As Topic 5 Notes - Alkanes

3/5

Topic /- – &l+anesRevision Notes

1) "eneral

• &l#anes are saturated hydrocarbons with general ormula C nH2n"2• 5aturated ) only single C0C bonds• Hydrocarbon ) contains C and H only• &l#ane molecules are non0polar so the only intermolecular orces are an

der 4aal7s orces

%) 0oiling points

• Boiling point increases with chain length ' more electrons more an der4aal7s orces between molecules

• Boiling point decreases as branching increases ' branched al#anes ha%eless sur ace area in contact so intermolecular orces are wea#er (orstraighter chains can pac# closer more an der 4aal7s orces betweenmolecules!

• *he /rst our al#anes are gases and are used as uels (methane ordomestic heating and coo#ing propane as 8-9 and in canisters orcamping cara%anning butane or cigarette lighters and in canisters!

• -etrol consists o li;uid al#anes with between 3 and < carbons

') Fractional $istillation

• Crude oil (petroleum! is a mi ture o many compounds most o which are

al#anes• Crude oil is separated into ractions many o which can be used directly asuels

• *he separation process is called ractional distillation. *his in%ol%es:o 5eparation is based on the diferent boiling points o al#aneso *he boiling points o al#anes depend on the si=e o the molecule

(and strength o an der 4aals orces!o aporised crude oil is ed into a column that is hot at the bottom

and cool at the topo 5maller molecules with lower boiling points come out at the top o

the column. 8arger molecules with higher boiling points come out atthe bottom

) Crac+ing

8/9/2019 As Topic 5 Notes - Alkanes

4/5

• Crude oil contains more long chain al#anes than are needed. Crac#ingbrea#s these al#anes down into products or which there is higher demand

• Crac#ing in%ol%es the brea#ing o C0C bonds in al#anes and thisre;uires a high temperature

!2ample3 C 14 %% C 1 6 C %$ecane octane ethene

• *hermal crac#ing gi%es a high proportion o al#enes. +t needs a hightemperature ( >>0?>> ° C! and a high pressure (@>>> #-a!

• *he al#enes rom thermal crac#ing are used to ma#e polymers andalcohols

• Catalytic crac#ing produces motor uels and aromatic hydrocarbons. +tneeds a =eolite catalyst a high temperature ( 3> ° C! and a slightpressure

5) Com-ustion

a) Introduction

Complete combustion re;uires a plenti ul supply o o ygen e.g.

C 5 1% 6 O % 5CO % 6 / %O

Combustion o ossil uels including al#anes results in the release ocarbon dio ide into the atmosphere+n a limited supply o air incomplete combustion occurs orming CO orC (soot!

C 5 1% 6 575O % 5CO 6 / %OC 5 1% 6 'O % 5C 6 / %O

+ncomplete combustion wastes petrol meaning more uel is needed

-) Catalytic converters

*he pollutants produced by car engines include carbon C carbonmono ide CO sulphur dio ide 5O 2 o ides o nitrogen 1O andunburnt hydrocarbonsCarbon mono ide and carbon come rom the incomplete combustion o the hydrocarbons in petrol and are to ic. Carbon is also a respiratoryirritant5ulphur dio ide is produced when traces o sulphur in the uel reactwith o ygen. 5O 2 causes acid rainO ides o nitrogen are produced when o ygen and nitrogen rom the airreact together due to the %ery high temperatures reached inside carengines. 1O 2 is to ic triggers asthma attac#s and orms H1O $ (i.e. acidrain! when it reacts with water and o ygen5mall amounts o hydrocarbons pass straight through a car enginewithout being burntCatalytic con%erters consist o a honeycomb o ceramic material coatedwith platinum palladium or rhodium (-t -d Ah!

*he honeycomb produces a large sur ace area on which reactions canoccur

8/9/2019 As Topic 5 Notes - Alkanes

5/5

Catalytic con%erters reduce the emission o CO and 1O by allowingthem to react together to ma#e harmless products

%CO,g) 6 %NO,g) %CO %,g) 6 N %,g)

c) "lo-al 8arming

• +n the troposphere (lowest le%el o the atmosphere! %arious gasesabsorb in rared radiation and #eep the atmosphere warm

• +n rared radiation is absorbed by C)O bonds in CO 2 O0H bonds in H 2Oand C0H bonds in methane. *he absorbed energy ma#es the bonds%ibrate

• +ncreased concentrations o greenhouse gases li#e CO 2 maycontribute to global warming because o the increased absorption o +Aradiation

d) &cid Rain

• Combustion o uels containing sulphur produces sulphur dio ide• &cid rain is ormed when 5O 2 dissol%es in water• 5O 2 which is acidic can be remo%ed rom ue gases using CaO which

is a base. *his is a neutralisation reaction.

#O % 6 CaO Ca#O '