Kimia Alkane

7/28/2019 Kimia Alkane

1/29

Alkanes

Alkanes are fully saturated hydrocarbons

- hydrocarbons have only Cs and Hs- saturated = all single bonds (max. # of Hs)

Alkanes have the general formula CnH2n+2

- example: C4H10 They can be straight-chained or branched

- example: CH3CH2CH2CH3 vs. (CH3)2CHCH3

Alkanes, and all other organic compounds, are

named according the the IUPAC system(International Union of Pure and AppliedChemistry)


2/29

Alkanes lin

e-angle formula:

a line represents a carbon-carbon bond and an angle

represents a carbon atom

a line ending in space represents a -CH3 group

hydrogen atoms are not shown in line-angle formulas

CH3CH

2CH

2CH

3CH3CH2CH3 CH3CH2CH2CH2CH3CH3CH2CH2CH3CH3CH2CH3 CH3CH2CH2CH2CH3

PentaneButanePropane PentaneButanePropane

Condensedstructural

formula

Line-angleformula

Ball-and-stick model


3/29


4/29

Naming Straight-Chain Alkanes and Alkyl Groups

The names of all alkanes end in -ane

The number of carbons in a straight-chain alkane isindicated by putting a prefix before the -ane

Examples: CH4 = methane

CH3-CH3 = ethane

CH3-CH2-CH3 = propane In branched alkanes the substituents (groups attached

to the carbon chain) are called alkyl groups

An alkyl group = an alkane with one H removed

Alkyl groups are named by changing the ending of thealkane name to -yl

Example: CH3-CH2- = ethyl


5/29


6/29


7/29

Classification of Carbons in Alkanes

Carbons can be classified by how many other Cs

are attached to them:-No Cs = methyl CH4

- 1 C = primary (1) CH3-CH3

- 2 Cs = secondary (2) CH3-CH2-CH3

- 3 Cs = tertiary (3) CH3-CH(CH3)2

- 4 Cs = quaternary (4) C(CH3)4


8/29

Naming Branched Alkanes

First find the longest chain of Cs (parent)

Number the Cs in parent chain (begin at endnearest 1rst branch point)

Identify substituents and number by point ofattachment to parent chain

Write full name

Example:

CH3-CH(CH3)-CH2-CH3 = 2-methylbutane

If there is more than one of a substituent a prefix isused and a number is given for each substituent

Example:

CH3-CH2-C(CH3)3 = 2,2-dimethylbutane


9/29

IUPAC Names1. The name for an alkane with an unbranched chain

of carbon atoms consists of a prefix showing the

number of carbon atoms and the ending -ane

2. For branched-chain alkanes, longest chain ofcarbon atoms is the parent chain and its name is

the root name

3. Name and number each substituent on the parent

chain; use a hyphen to connect the number to the

name

CH3CHCH3

CH3

2-Methylpropane

12

3


10/29

IUPAC Names

4. If there is one substituent, number the

parent chain from the end that gives the

substituent the lower number

CH3

CH3CH

2CH

2CHCH

35

2-Methylpentane(not 4-methylpentane)

12

34


11/29

IUPAC Names

5. If the same substituent occurs more than once,number the parent chain from the end that gives

the lower number to the substituent encountered

first

indicate the number of times the substituent occurs

by a prefix di-, tri-, tetra-, penta-, hexa-, and so on

use a comma to separate position numbers

CH3CH2CHCH2CHCH3

CH3 CH3

2,4-Dimethylhexane(not 3,5-dimethylhexane)

12

3

45

6


12/29

IUPAC Names

6. If there are two or more different substituents list them in alphabetical order

number the chain from the end that gives the lower

number to the substituent encountered first

if there are different substituents in equivalent positionson opposite ends of the parent chain, give the substituent

of lower alphabetical order the lower number

CH3CH

2CHCH

2CHCH

2CH

3

CH3

CH2CH

3

12

34

56

7

3-Ethyl-5-methylheptane

(not 3-methyl-5-ethylheptane)


13/29

IUPAC Names

7. Do not include the prefixes di-, tri-, tetra-,and so on, or the hyphenated prefixes sec-

and tert- in alphabetizing;

alphabetize the names of substituents first, andthen insert these prefixes

CH2CH3

CH3CCH2CHCH2CH3

CH3

CH34-Ethyl-2,2-dimethylhexane

(not 2,2-dimethyl-4-ethylhexane)

2 34

5

6

1


14/29


15/29


16/29


17/29

Constitutional Isomerism

Constitutional isomers:compounds that have thesame molecular formula but different structural

formulas

for the molecular formulas CH4, C2H6, and C3H8, only one

structural formula is possible; there are no constitutionalisomers for these molecular formulas

for the molecular formula C4H10, two constitutional

isomers are possible

CH3CH2CH2CH3 CH3CHCH3

CH3

Butane

(bp -0.5C)

2-Methylpropane

(bp -11.6C)


18/29

Haloalkanes

Haloalkanes have one or more halogens replacinghydrogen on an alkane

The halogens are numbered and named assubstituents

- F = fluoro

- Cl = chloro

- Br = bromo- I = iodo

If more than one halogen is present, they arenamed in alphabetical order

Example:

CH3-CH(Br)-CH(Cl)-CH3 = 2-bromo-3-chlorobutane


19/29

Cycloalkanes

Carbons can also bond together to form rings

Rings with only Cs, Hs and single bonds arecalled cycloalkanes

Cycloalkanes have the general formula CnH2n

The smallest is cyclopropane (C3H6)

- cyclopropane is an unstable molecule

- its forced to have bond angles of 120betweenthe Cs, while they would normally be 109 (thecarbons each have 4 electrons groups and should

be tetrahedral)

The only cycloalkanes with little or no strain arecyclopentane (C5H10) and cyclohexane (C6H12)

Most cycloalkanes are not flat because they prefertetrahedral geometry


20/29


21/29


22/29

Cis and Trans Isomers

Because cycloalkanes do not have free rotationaround the carbons, it matters on which side of thering a substituent is relative to other substituents

Two substituents on the same side (top or bottom)of the ring are called Cis

Two substituents on opposite sides of the ring arecalled Trans

Cis and Trans isomers are stereoisomers; theyhave the same molecular formula, and the atoms

are connected in the same order but are arranged ina different spacial orientation


23/29


24/29

Cyclohexane

HH

H

HH

H

(a) Ball-and-stick modelshowing all 12 hydrogens

axis through thecen ter of the rin g

H H

H

H

H

H

(b) The s ix equatorialC-H bonds (c) The six axialC-H bonds


25/29

Cyclohexane

the more stable conformation of a substituted

cyclohexane ring has substituent group(s)

equatorial rather than axial

CH3

Equatorial methylcyclohexane

CH3

Axial methylcyclohexane


26/29

Physical Properties of Alkanes and Cycloalkanes

Alkanes are nonpolar and are not soluble in water

They have low melting and boiling points due totheir weak intermolecular forces (dispersion)

They are also less dense than water

Alkanes are mostly obtained from crude oil

- the crude oil is fractionated based on boiling pt.- heavier fractions are often cracked, put underhigh heat and pressure, to obtain more gasoline

Alkanes are used extensively as fuels of various

types (for heating, cooking, driving, etc.)


27/29


28/29

Reactions of Alkanes and Cycloalkanes

Because they are nonpolar, and their covalentbonds are strong, alkanes and cycloalkanes are

relatively unreactive

The two types of reactions that they do undergoare combustion and halogenation

In combustion reactions, alkanes react withoxygen to form carbon dioxide, water and heat

CH4 + 2O2 CO2 + 2H2O + Heat

In halogenation reactions, halogens replace one or

more Hs on an alkane (a substitution reaction)CH4 + Cl2 (+ light or heat) CH3Cl + HCl

Mechanism of Alkane Halogenation


29/29

Mechanism of Alkane Halogenation These halogenations are radical reactions

Radicals are molecules (or atoms) that have one or more unpairedelectrons (the half-headed arrows represent movement of single electrons

1. Initiation:

H C H

H

H

Cl Cl + Light Cl + Cl

2. Propagation: + ClH C

H

H

+ H-Cl

H C

H

H

+ Cl ClH C Cl

H

H

+ Cl

3. Termination: H C

H

H

+ Cl H C Cl

H

H

Kimia Alkane

Documents

Transcript of Kimia Alkane