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Chemistry

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Centre for Foundation Studies, UTAR

Chapter Scopes
FHSC1124
Organic
Chemistry
Alkanes

• IUPAC Nomenclature / naming of aliphatic alkanes and cycloalkanes
• Physical properties
• Combustion reactions
• Free-radical substitution reactions
& mechanism
• Crude oil and “cracking”

Chapter 2

Introduction to Alkanes

IUPAC Nomenclature

• Simplest member of hydrocarbon family
• General formula of alkanes = CnH2n+2
• Alkanes = Paraffins
(hydrocarbons with general formula CnH2n+2)
• Aliphatic compounds: open chain / acyclic compounds • The names of alkanes end with suffix -ane.
• Saturated hydrocarbon: only have C−C & C−H single bonds & contain the maximum possible number of H per C.
3

IUPAC Rules
1. Select the longest continuous C chain as parent chain (use root word for the no. of
C)
2. Name each of the branch/substituents as an alkyl / aryl group
3. Number the C chain beginning from the end nearest to the branch
⇒ branch/substituents appear at the lowest no. possible

FHSC1124 ORGANIC CHEMISTRY

• IUPAC  International Union of Pure &
Applied Chemistry
• The IUPAC nomenclature system is a set of logical rules devised and used by organic chemists to name the organic compounds. Prefix − Parent − Suffix
What are the How many What family? substituents? carbons?

IUPAC Rules
4. Name each substituent according to its chemical identity & the no. of the C atom to which it is attached
⇒ For identical substituent, use the prefix di, tri… & write appropriate C no. for each substituent 5. Separate no. from no. by commas (,) & no. from letters by hyphens (-)
6. List the substituents alphabetically by name ⇒ di, tri…. don’t count

1

Centre for Foundation Studies, UTAR

Straight-Chain Alkyl Groups, R

Base Names
Root
Name
Meth
Eth
Prop
But
Pent

No. of
Carbons (n)
1
2
3
4
5

Root
Name
Hex
Hept
Oct
Non
Dec

No. of
Carbons (n)
6
7
8
9
10

Example: Naming Alkanes

1

2

3

4

Alkyl group
Name (abbreviation)
−CH3
Methyl (Me)
−CH2CH3
Ethyl (Et)
−CH2CH2CH3
Propyl (Pr)
−CH2CH2CH2CH3
Butyl (Bu)
−CH2CH2CH2CH2CH3
Pentyl
• Alkyl groups are named by replacing the
–ane ending of the parent alkane with an
–yl ending

Condensed structural formula

Carbon skeleton 1

2

3

4

5

6

1. Longest continuous C chain is 6 − hex
2. All C & H with single bonds − ane
3. Parent name: hexane
4. Two methyl groups on 2nd C & 5th C − dimethyl
5. Use 2,5-dimethylhexane

Cycloalkanes:
Cyclic Structures of Alkanes

FHSC1124 ORGANIC CHEMISTRY

PHYSICAL PROPERTIES OF ALKANES
Name
Methane
Ethane
Propane
Butane
Pentane
Hexane
Heptane
Octane
Nonane
Decane

BP (oC)
-161.7
-88.6
-42.2
-0.5
36.1
68.7
98.4
125.6
150.7
174.0

MP (oC) State at 25oC Density
-182.6
Gas
0.424
-172.0
Gas
0.546
-187.1
Gas
0.582
-135.0
Gas
0.579
-129.7
Liquid
0.626
-94.0
Liquid
0.659
-90.5
Liquid
0.684
-56.8
Liquid
0.703
-53.7
Liquid
0.718
-29.7
Liquid
0.730

2

Centre for Foundation Studies, UTAR

Boiling Points of Alkanes
Alkanes have relatively low bp
(e.g.: methane bp~ -161 oC) because:
Alkanes are non-polar molecules held by weak Van der Waals forces

Comparison of BP:
Branched-Chain & Straight-Chain

As the number of C ↑, molecules get bigger,
Van der Waals forces become stronger and more energy needed to break the bond

BP (K)

pentane

309.2

2-methylbutane
BP of alkanes increase as number of carbon increase because:

Isomer

301.0

2,2-dimethylpropane

282.6

The more branched the chain, the lower the boiling point tends to be.

Comparison of BP:
Branched-Chain & StraightChain

Reaction of Alkanes

The branched-chain isomer has a lower bp compared to straight-chain isomer:
• Branched-chain alkanes are short, fat & spherical in shape.
• This reduces the surface area that is exposed to Van der Waals forces.

Combustion / oxidation of
Alkanes

Free Radical
Substitution

• Less energy is needed to break the forces, bp decreases.
15

1. Combustion / Oxidation of Alkanes
• Alkanes burn in excess air or O2, to formed CO2 and H2O
CxHy +

y

 x + O
4 2



4

xCO2 +
4

y
2

H2O

CH4 +  1 + 4  O2 CO2 + 2 H2O


CH4 + 2O2
CO2 + 2H2O

FHSC1124 ORGANIC CHEMISTRY

2. Free Radical Substitution of Alkanes (Halogenation)
Alkanes react with chlorine or bromine:
• When the mixture is heated at 300 – 400 oC, OR
• At room temperature under the influence of UV light which act as catalysts.

light / heat
X−X +

C

H

C
X = Br, Cl

Halogens

X + H−X

Haloalkanes
/ alkyl halides

3

Centre for Foundation Studies, UTAR

Chlorination of Methane

Mechanism of Halogenation
• Chlorination or bromination of alkanes is by a free-radical substitution mechanism.
• Involved 3 steps:
Step 1: Initiation
Step 2: Propagation
Step 3: Termination

Mechanism of Halogenation:
Step 1: Initiation
• Heat or UV light cause the weak halogen bond to undergo homolytic cleavage to generate two bromine radicals and start the chain process.

Mechanism of Halogenation:
Step 2: Propagation
(a) Bromine radical (Br•) abstracts a H from methane to form HBr and a methyl radical (CH3•)
(b) The methyl radical abstracts a Br from another molecule of Br2 to form the methyl bromide and another bromine radical, which can then itself undergo reaction 2(a) creating a cycle that can repeat. 22

Mechanism of Halogenation:
Step 2: Propagation

(a)

Mechanism of Halogenation:
Step 3: Termination
• Various reactions between possible pairs of radicals allow the formation of ethane,
Br2 or the product, methyl bromide. These reactions remove radicals and do not perpetuate the cycle.

(b)

FHSC1124 ORGANIC CHEMISTRY

4

Centre for Foundation Studies, UTAR

Reactions of Cycloalkanes
• The reactions of cycloalkanes are generally the same as alkanes, i.e. free radical substitution (halogenation) & oxidation. Example: Halogenation of Cycloalkanes

+ Br2

Br + HBr

Crude Oil / Petroleum
• Alkanes occur naturally in petroleum and natural gas.
• Petroleum (brownish to black liquid) formed from dead organisms that sank to the bottom shallow seas and were subsequently covered by clay & silt.
• Pressure heating caused by burial changed these organic matter slowly, after million of years, into petroleum.
• Raw petroleum consists of a mixture of hydrocarbon of various sizes
⇒ separated by distillation before used.
26

Cracking of Alkanes
• Cracking involves the splitting of large alkane molecules into a mixture of smaller-sized alkanes and alkenes.

Cracking of Alkanes

Cracking of Alkanes
Catalytic Cracking

Catalytic Cracking
• Modern cracking uses zeolites as the catalyst. • Zeolites = complex aluminosilicates, and are large lattices of aluminium, silicon and oxygen atoms carrying a negative charge.
• The alkane is brought into contact with
C
the catalyst at a T of about 500° and moderately low P.

Thermal/Steam Cracking

FHSC1124 ORGANIC CHEMISTRY

5

Centre for Foundation Studies, UTAR

Thermal / Steam Cracking
• In thermal cracking, high T (typically in the range of 450° to 750° ) & P (up to
C
C about 70 atm) are used to break the large hydrocarbons into smaller ones.
• Thermal cracking gives mixtures of products containing high proportions of hydrocarbons with double bonds alkenes.

FHSC1124 ORGANIC CHEMISTRY

Upon learning this chapter, you should be able to:
• Use IUPAC nomenclature to name and draw correct structure of simple organic compounds with different functional groups.
• Select reagents to accomplish appropriate functional group transformations.
• Draw/write the products of organic chemical reactions. • Describe elementary mechanism of a particular organic reaction and write the reaction equation.
• Explain the physical properties of organic compounds. 6

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