ALKANES

CnH2n+2

Alkanes or CnH2n+2

C3H8

H H H

H–C–C–C–H

H H H

CH3CH2CH3

Chemical Formula

Structural Formula

Condensed Structural Formula

Alkanes: base unit

CH4

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 of C’s increases, boiling point ↑– molecules get heavier & it’s harder to change

into gas phase

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

A. Low melting point

B. High melting point

C. Soluble in polar solvents

D. 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

10Dec

9Non

8Oct

7Hept

6Hex

5Pent

4But

3Prop

2Eth

1Meth

# of C atomsPrefix

C5H12

C4H10

C3H8

C2H6

Meth + aneMeth + aneCH4

NameFormula

EthaneEthane

PropanePropane

ButaneButane

PentanePentane

Branched-chain alkanesBranched-chain alkanes

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

H

H–C–H

H H

H–C–C–C–H

H 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 in the formula, the more isomers more possible ways to arrange them

Isomers

• Different structures, different Different structures, different propertiesproperties

• Isomers have different chemical and physical properties

A note about isomers ---

• If comparing 2 structural formulas & you 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 the 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: to 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 # of C’s in longest chain• branchesbranches named inin prefixprefix

– Branches also named by number of C’s – “branchbranch” part of name ends in “yl”ends in “yl”

• location of branch shown by assignassigning #’s to C’s #’s to C’s in backbonein backbone

• number from end that gives lowest # for branch• if more than 1 of same type branch:

– use di, tri, tetra

H

H–C–H

H H

H–C–C–C–H

H H H

Branch: 1 carbon – methylmethyl

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

Methyl propaneMethyl propane

C4H10

CH3CH(CH3)CH3

Longest continuous chain has 3 carbon atoms = propanepropane

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

2-methyl hexane2-methyl hexane

C7H16

CH3CH(CH3)CH2CH2CH2CH3

A note about branches

• If have 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

3-methyl hexane

C7H16

CH3CH2CH2CH(CH3)CH2CH3

CH3(CH2)2CH(CH3)CH2CH3