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Alkanes
1. Genera Generall Formu Formula la : CnH2n+2 (number of atoms, n = 1,2,3......) 2. They They are are saturated hydrocarbons; each carbon atom is bonded to four other atoms by single covalent bonds. 3. The members members of the family, family, ending ending with name name “ane”. Number of carbon Name atoms (n)
Molecular formula
CnH2n+2
Physical Molar state at mass room (g mol-1) temperatu re
1
Methane
C1H2x1+2 = CH4
16
Gas
2
Ethane
C2H2x2+2 = C2H6
30
Gas
3
Propane
C3H2x3+2 = C3H8
44
Gas
4
Butane
C4H2x4+2 = C4H10
58
Gas
5
Pentane
C5H2x5+2 = C5H12
72
Liquid
6
Hexane
C6H2x6+2 = C6H14
86
Liquid
7
Heptane
C7H2x7+2 = C7H16
100
Liquid
8
Octane
C8H2x8+2 = C8H18
114
Liquid
9
Nonane
C9H2x9+2 = C9H20
128
Liquid
10
Decane
C10H2x10+2 = C10H22
142
Liquid
Consecutive members different different in molar mass is 14 g mol -1
4. Structural formulae of alkanes Name Methane
No. of isomers 0
Structural formulae and Name H │ H ─ C ─ H │ H
CH4
Ethane, C2H6
0
Propane, C3H8
0
Butane, C4H10
2
H H │ │ H ─ C ─ C ─ H │ │ H H H H H │ │ │ H ─ C ─ C ─ C ─ H │ │ │ H H H H H H H │ │ │ │ H ─ C ─ C ─ C ─ C ─ H │ │ │ │ H H H H n-butane
H H H │ │ │ H ─ C ─── C ─── C ─ H │ │ │ H H─ C ─H H │ H 2 - methyl propane Pentane, C5H12
3 H H H H H │ │ │ │ │ H ─ C ─ C ─ C ─ C ─ C ─ H │ │ │ │ │ H H H H H n-pentane H H H H │ │ │ │ H ─ C ─── C ─── C ─ C ─ H │1 │2 │3 │ 4 H H─ C ─H H H │ H 2-methyl butane
H │ H H ─ C ─ H H │ │ │ H ─ C ─── C ─── C ─ H │ │ │ H H─ C ─H H │ H 2,2-dimetyl propane Hexane, C6H14
5 H H H H H H │ │ │ │ │ │ H ─ C ─ C ─ C ─ C ─ C ─ C ─ H │ │ │ │ │ │ H H H H H H n-hexane H H H H H │ │ │ │ │ H ─ C ─── C ─── C ─ C ─ C ─ H │ │ │ │ │ H H─ C ─H H H H │ H 2 - methyl pentane
H H H H H │ │ │ │ │ H ─ C ─ C ─── C ─── C ─ C ─ H │ │ │ │ │ H H H─ C ─H H H │ H 3 - methyl pentane H │ H H ─ C ─ H H H │ │ │ │ H ─ C ─── C ─── C ─ C ─ H │ │ │ │ H H─ C ─H H H │ H 2,2 - dimethyl butane H │ H H H ─ C ─H H │ │ │ │ H ─ C ─── C ─── C ─── C ─ H │ │ │ │ H H─ C ─H H H │ H 2,3 - dimethyl butane
Physical Properties
Physical properties of alkanes i. cannot conduct electrity ii. less dense than water iii. dissolve in organic solvents, insoluble in water iv. low melting and boiling points Conclusion: - molecule held together by weak intermolecular forces - properties of covalent compound - gradually steady increase as the number of carbon in alkane increases
6. Steps to name branched alkanes; i. determined and named the long chains ii. determined and named the branch chain CH3 : methyl C2H5 OR CH2CH3 : ethyl C3H7 OR CH2CH2CH3 : prophyl iii. give number to the carbon atoms in long chain, which started from the nearest branched iv. The number for carbon atom which branched emerged from, must put before/infront the alkyl v. Named the branched first, followed by the named of long chains The word “ di, tri” is used if the branched chains is more than one
Chemical Properties Pg 38 Reactivity of alkanes 1. Not reactive/unreactive because saturated hydrocarbon 2. Did not decolourized purple solution of acidified potassium manganate(VII)
3. Did not decolourized reddish brown solution of bromin water 4. Neutral. Combustion of alkanes 1. In the presence of sufficient oxygen, alkanes burns to form carbon dioxide and water. – complete combustion Chemical equation: i.
CH4
+
O2
CH4 + 2O2
CO2
+
H2O
CO2 + 2H2O
ii.
C2H6
iii.
C4H10 + 13/2 O2
C6H14
iv.
+ 7/2
O2
+ 19/2 O2
2CO2
+ 3H2O
4CO2
+ 5H2O
6CO2
+
7H2O
Answers
i. CH4
+ 2O2
→
ii. C2H6
+
2 x C2H 6
+ 2 x 7 O2 2
2C2H6
7 O2 2
+
7 O2
CO2
+ 2H2O
2CO2 +
3H2O
2x 2CO2 + 2x 3H2O 4CO2 +
6H2O
iii.
C4H10 + 13/2 O2
iv.
C6H14 + 19/2 O2
4CO2 + 5H2O 6CO2 + 7H2O
C3H8 C5H12 C7H16 C8H18 C9H20 C10H22 2. If there is insufficient oxygen, carbon monoxide or carbon may be formed – incomplete combustion
i. CH4 + 3/2 O2 2CH4 + 3 O2
CO + 2H2O 2CO + 4H2O
ii. CH4 + O2
C + 2H2O
Halogenation
The reaction is between alkane dan chlorine. Takes place under sunlight/ultra violet light. Carbon-hydrogen bonds broken and new carbon-halogen bonds are formed. One or more hydrogen atoms in alkanes molecule may be subtituted by halogen. 4. Halogenation is substitution reaction.
1. 2. 3.
Chlorination of methane First stage; H │ H ─ C ─ H +Cl-Cl → │ H
