Topic: Alkanes

Do Now: Draw the possible bonds a single carbon atom can have if it has a bonding

capacity of 4

Alkanes• Homologous series of saturated hydrocarbons• Release energy when burned

• CnH2n+2 – all have this general formula

CH4 H

H–C–H

H

C2H6 H H

H–C–C–H

H H

Saturated hydrocarbons: organic compounds containing only Single bonds – can’t add any more Hydrogens

Properties of Alkanes• Low Reactivity

–Except readily undergo combustion (fuels)• Nonpolar – don’t dissolve well in water• Low melting & low boiling points

–both increase with molecular mass• High vapor pressures

Properties of Alkanes

• Change systematically with # of C’s

• As # of C’s increases, boiling point increases– molecules get heavier so harder to change into gas

phase

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 = A

A. CH4

B. C2H6

C. C3H8

D. C4H10

Which of the following compounds has the highest boiling point?

Correct answer = D

nonpolar coval cmpd:

• bp depends on strength of van der Waals interactions

• van der Waals forces ↑ as size of molecule ↑

• bigger molecule = bigger electron cloud

Alkanes (CnH2n+2)

C3H8

H H HH–C–C–C–H

H H H

CH3CH2CH3

Chemical Formula

Structural Formula

Condensed Structural Formula

straight-chain alkane

C4H10

H H H HH–C–C–C–C–H

H H H H

Or CH3CH2CH2CH3

Naming straight-chain Alkanes

• Name: describes molecule so can draw it

• all alkanes have the suffix –ane• prefix: tells # of C’s

10Dec

9Non

8Oct

7Hept

6Hex

5Pent

4But

3Prop

2Eth

1Meth

# of C atomsPrefix

C5H12

C4H10

C3H8

C2H6

Meth + aneCH4

NameFormula

Ethane

PropaneButanePentane

Naming Branched-Chain Alkanes

• goal of name is to describe molecule so you can draw it

Naming branched-chain alkanes1. Find longest continuous chain of C atoms

- Bends don’t matter!

2. base name derived from # C’s in continuous chain

9 carbon chain = nonane

3. Branches named first and in alphabetical order * to name branches, count # C atoms & add “yl” to prefix name* If more than one of same type of branch - use di, tri, etc.

ethyl

ethyl

ethylmethyl

propyl

triethyl, methyl, propyl

Assign number to Branches based on longest carbon chain

* you want numbers to add up to the lowest #

ethyl

ethyl

ethyl

methyl

propyl

1

23456

789

4, 4, 5, 5, 6 = add up = 24

Assign number to Branches based on longest carbon chain

* you want numbers to add up to the lowest #

ethyl

ethyl

ethyl

methyl

propyl

1234 5 6 7

8

9

4, 5, 5, 6, 6 = add up = 26

4. Put it all together

ethyl

ethyl

ethyl

methyl

propyl

1

23456

789

3. 4, 4, 5, 5, 6

2. triethyl, methyl, propyl

1. nonane

nonane4,4,6-triethyl 5-methyl 5-propyl

HH–C–H

H HH–C–C–C–H

H H H

2-methyl propaneNo because if methyl was anywhere else it wouldn’t be propane

Longest continuous chain has 3 carbon atoms – propane

1 2 3

Methyl 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 1

4 3

6

5

2

Longest continuous chain has 5 C atoms: pentane

Branch: 1 carbon = methylBranch: 12 carbon = ethyl

Branch located at C-2 and 3

3-ethly 2-methyl pentane

C8H18

CH3CH(CH3) CH(CH2CH3)CH2CH3

Branched-chain alkanes

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

HH–C–HH H

H–C–C–C–HH H H

ISOMERS