ALKANES

CnH2n+2

Alkanes or CnH2n+2

C3H8

H H HH–C–C–C–H

H H H

CH3CH2CH3

Chemical Formula

Structural Formula

Condensed Structural Formula

Alkanes: base unitCH4

CH3CH3 or C2H6

CH3CH2CH3 or C3H8

CH3CH2CH2CH3 or C4H10

CH3CH2CH2CH2CH3 or C5H12

Difference between each is CHCH22

Properties of Alkanes

• Change systematically with number of C’s

• As number C’s increases, boiling point ↑– molecules get heavier

Properties of Alkanes

• Low Reactivity– except readily undergo combustion – fuels

• Nonpolar – Don’t dissolve well in water• Low mp’s & bp’s

– mp & bp ↑ with ↑ molecular mass• High vapor pressures

– evaporate easily

A. Low melting pointB. High melting pointC. Soluble in polar solventsD. Insoluble in nonpolar solvents

Which property is generally characteristic of an organic compound?

Correct response = ACorrect response = A

A. CH4

B. C2H6

C. C3H8

D. C4H10

Which of the following compounds has the highest boiling point?

Correct answer = D Correct answer = D

bp depends on strength of forces holding molecules bp depends on strength of forces holding molecules together [forces together [forces ↑↑as size of molecule as size of molecule ↑]

Naming Hydrocarbons

Naming straight-chain Alkanes

• Name describes molecule so can draw it

• All alkaneanes have the suffix –aneane• prefix tells how many C’s

10Dec9Non8Oct7Hept6Hex5Pent4But3Prop2Eth1Meth

# of C atomsPrefix

C5H12

C4H10

C3H8

C2H6

Meth + aneMeth + aneCH4

NameFormula

EthaneEthane

PropanePropaneButaneButanePentanePentane

Branched-chain alkanesBranched-chain alkanes

Beginning with butane, C4H10, more than 1 way to arrange the atoms

HH–C–HH H

H–C–C–C–HH H H

Branched alkane- can’t link all C’s without lifting pencil off paper

Methyl propane

IsomersIsomers

• Compounds with same molecular formula but different structural arrangement

• more C atoms more isomers (more possible ways to arrange them)

Isomers

• Different structures = different Different structures = different propertiesproperties–different chemical & physical properties

A note about isomers ---

• comparing 2 structural formulas: if can superimpose them, then not isomers = same molecule!

• If can rotate or flip one of structural formulas & superimpose it on the other, they are not isomers – they are same molecule!

These diagrams represent the same molecule

Represents bond to H

These diagrams represent the same molecule

These diagrams represent the same molecule

Two different molecules!

Naming Branched

Hydrocarbons

Naming Branched-Chain Alkanes

• goal of name: describe molecule so can draw it

Naming branched-chain alkanes• find longest continuous chainlongest continuous chain (backbone) of C

atoms (Bends don’t count!)• base namebase name derived from # C’s in longest chain• branchesbranches named firstfirst

– Branches named by # C’s – “branchbranch” name ends in “yl”ends in “yl”

• assignassign #’s to C’s in backbone#’s to C’s in backbone • # from end so lowest # given to branch• more than 1 same type branch:

– use di, tri, tetra

HH–C–HH H

H–C–C–C–HH H H

Branch: 1 carbon – methylmethyl

Branch at C #2:- can’t be on any other C so doesn’t need #

Longest continuous chain has 3 carbon atoms = propanepropane

11 22 33

Methyl propaneMethyl propane

C4H10

CH3CH(CH3)CH3

HH–C–H

H–C–H H H HH–C–C–C–C–H H H H H–C–H H

Longest continuous chain: 6 C’s = hexanehexane

branch is 1 carbon long – methyl methyl.

branch located at C #22

C7H16

CH3CH(CH3)CH2CH2CH2CH3

33 11

66

55

4422

2-methyl hexane2-methyl hexane

A note about branches

• If several branches with same # C’s can condense name a bit

• 2-methyl 3-methyl pentane becomes 2,3-dimethyl pentane

• every branch must have a #

H

H – C – H

H H HH – C – C – C – C – H H H H–C–H H–C–H H H

Longest continuous chain has 6 C’s: hexane

-branch 1 C = methyl-branch located at C #3

C7H16

CH3CH2CH(CH3)CH2CH2CH3

3-methyl hexane