alkanes lecture 2014 final student
DESCRIPTION
NYJC chemTRANSCRIPT
-
H2 Chemistry 9647 Alkanes NYJC 2014
Page1of17
Alkanes Lecturers: Ms Joanne Low and Mrs Zhou (Mdm Tan Shuyun) Contents Alkanes (exemplified by ethane)
(i) Free-radical reactions Hydrocarbons as fuels Learning Outcomes Candidates should be able to: (a) Recognise the general unreactivity of alkanes, including towards polar reagents
(b) Describe the chemistry of alkanes as exemplified by the following reactions of ethane:
(i) Combustion (ii) Substitution by chlorine and by bromine
(c) Describe the mechanism of free-radical substitution at methyl groups with particular reference to the initiation, propagation and termination reactions
(d) Recognise the environmental consequences of: (i) Carbon monoxide, oxides of nitrogen and unburnt hydrocarbons arising from the
internal combustion engine and of their catalytic removal (ii) Gases that contribute to the enhanced greenhouse effect
1. Introduction 1.1 Alkanes
belong to a homologous series of hydrocarbons (contain C and H atoms only) are ____________ (only single bonds between atoms, hence they contain maximum
number of hydrogens per carbon atoms) are combustible but unreactive There are two homologous series of alkanes:
Aliphatic alkanes (open-chained alkanes) Alicyclic alkanes (closed-chained alkanes) General formula: CnH2n+2 General formula: CnH2n E.g. CH3CH2CH2CH2CH3 Pentane C5H12
E.g. Cyclobutane C4H8 Cyclopentane C5H10
-
NH2 Chemistr
1.2 Nomen No. of C atoms
1
2
3
4
4
5
5
6
1.3 Alkyl g formed named
Gen
C
ry 9647
nclature
IUPAC NMolecular
MethaCH
EthaC2H
PropaC3H
ButaC4H
2-methylpC4H
2,2-dimethyC5H
cyclopenC5H
cyclohexan
groups
d when ond by replac
Aliphatic Aneral formu
CH3 Metha
CH3CHEthan
CH3CH2CPropan
CH3CH2CHButan
Name/ formula
ane 4
ne H6
ane H8
ne 10
propane 10
ylpropane 12
ntane 10
ne C6H12
e of the hycing ane b
Alkane la: CnH2n+2
H ne 2 H
ne CH2 H ne 2CH2 H
ne
A
Condeform
CH
CH3C
CH3CH
CH3 (CH
CH(C
C(CH
H2C
H2C
HC
H2C
H2CCH
HC
ydrogen of by yl
Ge
Alkanes
Page2of17
ensed mula
H4
CH3
H2CH3
H2)2CH3
CH3)3
H3)4
H2C
CH2
CH2
CH2
CH2
CH2
H2C
the alkane
Alkyl grneral formu
CH3 Meth
CH3CHEthy
CH3CH2CProp
CH3CH2CHButy
Displayed
CH
C
H
H
H
C
H
H
H
es is remov
roup ula: CnH2n+1 yl
H2 yl CH2 yl
H2CH2 yl
C C
H
H
H
H
H
C
CC
H
HH
H
H
C
CC
CH
H
H
H
HH
d formula
C H
H
H
C H
H
H
C C
H
H H
H
H
ved
AGen
C
H
C
H
H
H
H
C
C
C
HH
H
HH
CC
CC
HH
H
H
H
H
Melting point /oC
182
183
190
138
160
17
94
7
Abbreviationeral symb
Me
Et
Pr
Bu
NYJC 201
C Boiling
point /oC
164
88
42
0.5
12
10
49
81
on ol: R
4
C
-
gH2 Chemistr
2. Bondi Bonding a
C has 4 C atom
configu Shape
2.1. Hybri
W
All the Hence Each c
atomic
C* (hyb
How?
1. The2. Dur
form3. The4. Bon
atom
groundstate
ry 9647
ing in Al
and molec
4 valence ms share euration. of the mol
dization o
What is hybThe comolecuHybridiview. Iorbital
carbons inalkanes h
carbon is aorbitals)
bridised s
e electron iring hybridm four sp3
e four sp3 hnding will tms.
eC heat
kanes
cular struc
electrons aelectrons b
lecule with
of carbon
bridizationncept of h
ular orbitalsisation dest involves s.
n alkanes aave carbonable to fo
state):
in the 2s odisation, on3 hybrid orhybrid orbitthen occur
excitedst
A
cture of al
and it has by forming
respect to
in alkanes
n? ybridisatios for molecscribes the mixing of
are sp3 hybn with tetrarm four sp
rbital first gne 2s and rbitals. tals form ar when the
1s fo
tateC he
Alkanes
Page3of17
lkanes
electron co4 covalen
o carbon: _
s
on is usefucules. bonding a
f atomic o
bridised. ahedral arrp3 hybrid o
gets excitethree 2p
a tetrahedre sp3 hybr
our sp3 hyb
hybreat
onfigurationt bonds to
_________
ul in explai
atoms fromrbitals to f
rangementorbitals (by
ed and it isatomic orb
ral arrangerid orbitals
brid orbitals
ridisedstate
n: _______o achieve
______
ining the s
m an atomsform new
t of CC ay mixing o
s promotedbitals are h
ement. (Booverlap w
s
C
_______. noble gas
hape of
s point of hybrid
and CH one 2s an
d to the 2p hybridised
ond angle =with orbita
NYJC 201
s electronic
bonds. d three 2p
orbital. (mixed) to
= 109o) als of othe
4
c
p
o
r
-
H2 Chemistr Recall: Ov Sigma Pi () All sin Example: During
form __ Each _ four C
Example: The C The tet
___ orb The re
___ orb
s orbital
ry 9647
verlap of oa () bondsbonds aregle bonds
Methane
hybridisat_______ h___ hybrid H ______
Ethane (C
atom formtrahedral cbital from emaining __bital of hyd
l of H
orbitals tos are forme formed bys are ____
(CH4)
tion, _____hybrid orborbital will___are for
C2H6)
s four sp3carbon atoeach C ato__ orbitals drogen ato
A
o form boned by ____y _______
_________
____ and _itals. overlap __
med.
3 hybrid oroms form aom.
of carbon m.
non
Alkanes
Page4of17
nds (Chem_________
______ove_ bonds.
________
_______ w
rbitals. a ________
form ____
ne sp3 orbitne sp3 orbi
mical bond_ overlap o
erlap of orb
atomic orb
with one of
_____ by
______ by
tal of C tal of C
ding chaptof orbitals bitals.
bitals are h
f the __ orb
_________
________
ter)
hybridised
bitals of fou
__ overlapp
__ overlap
NYJC 201
(mixed) to
ur H atoms
ping of one
ping with a
4
o
s.
e
a
-
H2 Chemistry 9647 Alkanes NYJC 2014
Page5of17
3. Physical Properties 3.1 Boiling point and melting point Alkanes have relatively ______ boiling and melting points.
Alkanes are non-polar: C H bond is considered non-polar as C and H only differ slightly in electronegativity. [electronegativity of C(2.5) and H(2.1) are similar]
Have a ________________________ consisting of __________________ held together by ______________________________.
Boiling or melting involves overcoming the weak van der Waals forces between the alkane molecules.
Boiling point generally ____________ with an ____________ number of C atoms.
C1 C4: gas C5 C17: liquid >C18: solid
- When Mr increases, number of electrons ____________. - Larger electron cloud becomes __________________. - Strength of intermolecular van der Waals forces ____________. - __________________ is required to overcome the van der Waals forces.
The greater the degree of branching (with the ______ number of C atoms), the
______ the boiling point.
Name pentane 2-methylbutane 2,2-dimethylpropane
Structure CH3CH2CH2CH2CH3
CH3CHCH2CH3
CH3 CH3CCH3
CH3
CH3 Boiling point /oC 36 28 10 Melting point /oC 130 160 17
- Highly branched alkanes are more ____________ in shape. - ________________________ of contact between molecules. - Strength of intermolecular van der Waals forces decreases. - Less energy is required to overcome the weak intermolecular forces.
The trend for melting point less regular than that of boiling point
- Branched alkanes can have lower or higher melting points than the straight chain alkanes depending on the ____________ of the molecules in the solid
- Highly symmetrical branched alkanes allow the molecules to be packed more efficiently in the solid and hence have unusually high melting point
*at room temperature
-
H2 Chemistry 9647 Alkanes NYJC 2014
Page6of17
3.2 Solubility Alkanes are: soluble in non-polar (organic) solvents like CCl4, benzene and ether (R-O-R).
- They can form van der Waals interactions with the non-polar solvent. - The energy released during the formation of van der Waals forces with the non-
polar solvent is enough to overcome the van der Waals forces between the alkane molecules and the van der Waals forces between the solvent molecules.
insoluble in polar solvents like water. - They can only interact with water molecules via weak van der Waals bonds - the energy released during the formation of weak van der Waals forces with
water is not enough to overcome the strong hydrogen bonds between water molecules.
and water molecules
strong hydrogen bonding between water molecules
water layeralkane layer
weak interactions between alkane
weak van der Waals between alkane molecules
+
+
+
+ +
+
+
+
-
H2 Chemistry 9647 Alkanes NYJC 2014
Page7of17
4. Chemical Properties Alkanes are saturated and are generally unreactive because:
(i) The C-H bond is ____________ They have no centres of electrical charge to act as electrophiles or
nucleophiles to attract polar reagents like H+, OH- or MnO4-. (ii) The C-C and C-H bonds are relatively ____________
C C: 350 kJ mol-1 C H: 410 kJ mol-1
Alkanes do, however, react with oxygen and halogens under appropriate conditions,
like in the ________________________ or ____________.
Alkanes undergo two main types of reactions: (i) Combustion (ii) Free Radical Substitution
(i) Combustion Complete combustion
Alkanes burn in ____________ oxygen to form ____________ and water. This reaction is very exothermic, which accounts for their use as fuels.
Alkanes burn with a non-luminous blue flame with little or no soot if combustion is complete.
General equation: CxHy + (x + 4
y ) O2 x CO2 + 2y H2O
e.g. Complete combustion of hexane:
C6H14 (l) + 192
O2 (g) 6CO2 (g) + 7H2O (l)
Incomplete combustion
In a ____________ supply of oxygen, alkanes burn to form ____________, water and soot (C).
e.g. Incomplete combustion of methane:
2CH4 + 3O2 2CO + 4H2O CH4 + O2 C + 2H2O (soot) 4CH4 + 5O2 2CO + 2C + 4H2O (soot)
-
H2 Chemistry 9647 Alkanes NYJC 2014
Page8of17
(ii) Substitution
Alkanes react with halogen (e.g. Cl2 and Br2) to form halogenoalkanes (alkyl halides) when irradiated with ultraviolet light or heated.
No reaction takes place in the dark. The mechanism for the halogenation reactions of alkanes is known as
______________________________.
Type of reaction free radical substitution
Equation CxHy (g) + Cl2 (g) CxHy-1Cl (g) + HCl (g) Reagents Cl2 Conditions
Heat may be supplied to initiate the reaction. Reaction proceeds very slowly at room temperature.
Product chloroalkanes Observations Yellowish-green colour of Cl2 decolourises.
White fumes of HCl produced. Example: Write a balanced equation for the reaction of propane with bromine. Indicate clearly the conditions and observations in this reaction. Conditions: Observations: Note: - Solvent is CCl4 (used when alkane and halogen is present in different phases)
Example: C3H8 (l) + Br2 (g) - Bromination takes place less readily than chlorination because bromine is less
reactive than chlorine. (Refer to Group VII) - Fluorination is dangerously exothermic while iodination is slow and reversible.
-
H2 Chemistr
5. React
A ccrea
Bon
The
i)
ii) 5.1 Homo
5.2 Hetero
ry 9647
ion mec
chemical rated.
nd cleavag
ere are two
Homol Hetero
lytic FissiHomolytic _________When the electrons. Free radic[Recall: A A free radtendency t
olytic fissiHeterolytic_________When the bonding e
hanism
reaction ta
ge or bond
o types of b
lytic fissioolytic fissi
ion fission ten
_____ electbond brea
cals are fofree radica
dical is extto pair up w
ion c fission te
_____ electbond brea
lectrons to
A
akes place
fission is t
bond fissio
on on
nds to occutronegativaks, each
ormed in hoal is an atoremely reawith anothe
ends to octronegativaks, the mo form an a
Alkanes
Page9of17
e when old
the breakin
on required
ur when thvities. of the bon
omolytic fisom or moleactive becer electron
ccur whenve than the
more electroanion. The
+
d bonds a
ng of chem
d for organ
e two bond
nded atom
ssion. ecule with aause the u
n from anot
one of the other. (e.onegative counterpa
are broken
mical bonds
ic chemistr
ded atoms
s takes __
an unpaireunpaired ether specie
he two bong. polar boatom take
art become
Half arr
Depictsof a sin
Full arrow
Depicts thof an elec
n and new
s.
ry mechan
are identic
_____ of th
ed electronelectron haes.
nded atomonds)
es _______es a cation
row :
s the movengle electr
w :
he movemctron pair.
NYJC 201
w ones are
nisms:
cal or have
he bonding
. E.g. Cl]as a strong
ms is much
______ then.
ement ron.
ment .
4
e
e
g
] g
h
e
-
H2 Ch
5.3 F
Initiat
Cl
Propa Cl + CH3C
Termi CH3C Cl + CH3C
emistry 9647
ree-Radical S
Mtion
Clu.v.
agation
CH3CH2H
H2 + ClCl
ination
H2 + Cl
Cl
H2 + CH3CH2
uv
ubstitution (F
Mechanism
Cl Cl
CH3CH2
CH3C
CH3CH2
ClCl
2
Alkanes
RS) Mechanis
2 + HCl
CH2Cl + Cl
2Cl
CH3CH2CH2CH
sm
Homoly
Why is CH3CH C-H bo Note: B The c
hydro A high
Why i Cl + The f
forma Note: The e
radica The tw
destro
H3
The c
Page10of17
ytic fission of Cl
this step not favH3 CHonds in alkanes aBE(Cl-Cl) = 242
chlorine radical gen chloride mohly reactive meth
s this step not fa CH3CH2H formation of a ation of a H-Cl bo
BE(H-Cl) = 431
ethyl radical reaal. wo steps are reoyed i.e. termina
chain reaction te
NYJC 2014
D
Cl bond under
vourable? H3CH2 + H are not broken akJmol-1, BE(C-H
removes a hyolecule. hyl radical is pro
avourable? CH3C-Cl bond is leond in the above kJmol-1, BE(C-C
cts with a chlor
epeated till all thation step.
rminates when t
4
Description
UV radiation to f
as the C-H bond H) = 435 kJmol-1
ydrogen atom f
oduced.
CH2Cl + Hess exothermic e mechanism. Cl) = 340 kJmol-
rine molecule to
he reactants hav
two free radicals
form free Cl rad
is much stronge
from a methan
and hence les
-1
o produce chloro
ve been used u
s collide with eac
dicals.
er than the Cl-C
e molecule to
ss favourable th
oethane and a c
p or when radic
ch other.
l bond.
form a
han the
chlorine
cals are
-
H2 Chemistry 9647 Alkanes NYJC 2014
Page11of17
Example The chlorination of propane in the presence of a limited amount of Cl2 forms 2-chloropropane as a possible product. Suggest the mechanism of the reaction.
Example 1-cholorocyclohexane can be made from cyclohexane. Describe the reaction mechanism of its formation.
-
H2 Chemistr
5.4. ProblIn reality, aof the follo 5.4.1 Mult Depen
for rea The re
CH3Cl CCl4 p To avo
(This m
ExamplA possi1,1-dich Mechan Initiatio
Cl CPropag
Termina
(**Write a
ry 9647
ems with a pure sam
owing prob
ti-substitunding on thaction, a meaction of C
predominpredominatoid multisumaximises
le ble produchloroethan
nism: Free
on
Cl Cuv/ heat
ation
ation
at least 3, in
Free-Radmple of thelems:
ution he relative ixture of pCH4 and C
ates whentes when t
ubstitution, s the proba
ct for the cne. Sugges
Radical S
Cl + Cl
ncluding for
P
ical Subst chloroalka
amounts oroducts wi
Cl2 is able to
n there is ehere is excuse an __
ability that C
chlorinatiost the mec
Substitutio
rmation of t
Alkanes
Page12of1
titution ane (or bro
of halogen ll be obtaino yield a m
xcess CH4cess Cl2. _________Cl attacks
n of ethanchanism of
on
he halogen
7
omoalkane
and alkanned. mixture of C
4.
_________s CH4.)
ne in the pthe reactio
, product an
e) is seldom
e present
CH3Cl, CH2
_____ or a
presence oon.
nd one poss
m obtained
and the tim
2Cl2, CHC
________
of excess
sible side p
NYJC 201
d because
me allowed
l3 and CCl4
_________
chlorine is
roduct)
4
d
4:
_.
s
-
H2 Chemistry 9647 Alkanes NYJC 2014
Page13of17
5.4.2 Isomeric Products In the case of alkanes with 3C, isomeric products are formed depending upon which
H atom is replaced.
Expected Ratio: 10% 90% A simplified way to obtain the expected ratios is to count the number of hydrogens that
have the same chemical environment which will lead to the formation of a specific product. (probability)
Example: Draw all the possible monosubstituted products when a mixture of 2-methylbutane and bromine is allowed to react in the presence of sunlight. State the ratio in which they are formed.
Br
C C
H
H
H
C
H
H
C
H
H
C
H
H
H
H :
H
C C
H
H
Br
C
H
H
C
H
H
C
H
H
H
H :
H
C C
H
H
H
C
H
Br
C
H
H
C
H
H
H
H
a) Draw the displayed formula of another isomer of C5H12 which reacts with bromine to form only one monobrominated product. Ans:
C C
Hd
Hd
Hd
Ha
C
Hb
C
Hb
Hc
Hc
Hc
C HdHd
Hd
-
H2 Chemistry 9647 Alkanes NYJC 2014
Page14of17
6. Environmental Concern (Refer to Heterogenous catalysis in Kinetics, Page 30) There are several environmental concerns regarding the use of hydrocarbons as fuels.
(i) Pollutants such as carbon monoxide, oxides of nitrogen and unburnt hydrocarbons (soot) found in car exhaust can be removed from the car engines using catalytic converter.
Pollutants Formation in car engine Environmental/ Health Impact
Reaction for removal in catalytic converter
Carbon monoxide
Incomplete combustion of fuel
Combines irreversibly with haemoglobin and makes it ineffective as oxygen carrier in the human body suffocation or blood poisoning
Conversion of CO to CO2 2CO(g) + O2(g) Pt 2CO2(g)
Oxides of nitrogen (NO2, NO)
Reaction of N2 with O2 at high temperatures
Catalyzes formation of acid rain Forms smog
Conversion of NO to N2 2NO(g) Pt N2(g) + O2(g) 2NO(g) + 2CO(g) N2(g) + 2CO2(g)
Unburnt hydrocarbon
Incomplete combustion of fuel
Forms smog Oxidation of unburnt hydrocarbon to CO2 and H2O e.g. 2C8H18(l) + 25O2(g) Pt 16CO2(g) + 18H2O(l)
Note:
- The catalyst in the converters would be poisoned by the lead present in petrol. Therefore, cars fitted with catalytic converters must use unleaded petrol.
- A honeycomb structure is used to maximise the surface area on which catalysed reactions can take place.
(ii) Carbon dioxide and methane are greenhouse gases responsible for global warming
and climate changes. One of the major sources of carbon dioxide comes from burning of fossil fuels for energy production; hence there is a need to find alternative fuels for mankind.
-
H2 Chemistry 9647 Alkanes NYJC 2014
Page15of17
Summary of important concepts in Alkanes
Physical Properties of Alkanes: Non-polar Consisting of molecules held together by weak intermolecular van der Waals forces. Soluble in non-polar (organic) solvents Insoluble in polar solvents Boiling point increases with number of carbons
o Number of electrons in the molecule increases. o Polarisability increases, o Strength of intermolecular van der Waals forces increases. o More energy is required.
Branching decreases the boiling point. o More spherical in shape o Smaller surface areas of contact between molecules o Strength of intermolecular van der Waals forces decreases o Less energy is required
Chemical Properties of Alkanes: Alkanes are unreactive because the C-H bond is non polar and relatively strong. Carbon atoms in alkanes form four sp3 hybrid orbitals from hybridization of one 2s and three 2p
orbitals. They form 4 sigma bonds () via head-on overlap of orbitals. Complete combustion of alkanes produces carbon dioxide(CO2) and water(H2O). Alkanes undergo reactions by a mechanism called free-radical substitution in the presence of
ultraviolet light or heat.
o In reaction of ethane with bromine, the 3 stages in the mechanism are (1) Initiation
Equation: Cl Clu.v. Cl Cl
(2) Propagation Equations:1st step - Cl 2CH3 CH2H H HClCH3CH2
2nd step - Cl Cl ClCH3CH2 CH3CH2 Cl
(3) Termination Equations:
2 Types of Bond Fission
Occurs between atoms of _______ electronegativities
Electrons in bond broken goes to Arrows Products
(1) Homolytic fission Similar One to each atom
Radicals
(2) Heterolytic fission Different
Both electrons go to more EN atom
Cation + Anion
CH3CH2 Cl CH3CH2 Cl
Cl Cl Cl ClCH3CH2 CH3CH2 CH2CH3CH3CH2
-
H2 Chemistry 9647 Alkanes NYJC 2014
Page16of17
Annex (For additional reading) A1. Cracking Cracking is a process of breaking down large alkane molecules into smaller alkanes and alkenes. There are two processes we can split alkane chains: By high temperature (about 800oC) and pressure, known as thermal cracking. By catalyst (Al2O3 and SiO2, 450 oC), known as catalytic cracking. For example, C10H22 C4H8 + C6H14 C14H30 C2H4 + C12H26
[Note TYS Pg 118, Qn 5a (N2005/III/8 Either) is no longer in syllabus]
A2. Hydrocarbons as Crude Oil Fractional Distillation Petroleum is a mixture of a very large number of different hydrocarbons; the most commonly found molecules are alkanes (linear or branched), cycloalkanes and aromatic hydrocarbons. Raw oil or unprocessed ("crude") oil is not useful in the form it comes in out of the ground. To make it useful, it must be separated into its components by fractional distillation as shown in Figure 1.
Figure 1. Fractional distillation of crude oil. The fractionating column is cooler at the top than at the bottom because the fractions at the top have lower boiling points than the fractions at the bottom.
-
H2 Chemistry 9647 Alkanes NYJC 2014
Page17of17
Crude oil is fractionally distilled to give the following fractions. Fraction Length of Carbon
chain Uses
Refinery
gases
C1-C4 Fuel; domestic heating, gas cookers
Gasoline C5-C12 Fuel in internal combustion engines
Kerosene C12-C18 Fuel for jet engines
Diesel Oil C18-C25 Fuel for transport and industrial
heating
Residue > C25 paraffin wax, lubricating oil, petroleum
jelly, bitumen
A3. Octane number (octane rating)
In a cylinder of a motor car engine, a mixture of petrol vapour (mostly C5 to C10 alkanes) and the air is ignited by an electric spark, producing an explosive reaction which drives the piston down.
Petrol rich in straight-chain alkanes (e.g. heptane) ignites very readily and explodes rapidly, causing knocking of the engine and inefficient combustion.
Combustion of branch-chain alkanes (e.g. 2,2,4-trimethylpentane) is much smoother and more controlled. It is a more efficient fuel and less likely to cause knocking.
A numerical representation of the antiknock properties of motor fuel, compared with a standard reference fuel.
Heptane is assigned an octane number of 0. 2,2,4-trimethylpentane (iso-octane) is assigned an octane number of 100. The octane number of a petrol is found by comparing its performance with a mixture of
heptane and 2,2,4-trimethylpentane. The octane number of a sample of fuel is determined by burning the gasoline in an
engine under controlled conditions, e.g., of spark timing, compression, engine speed, and load, until a standard level of knock occurs.
Gasoline that have high octane numbers denotes that they have good anti-knock properties
Reference texts 1. Understanding Chemistry for Advanced level, Ted Lester, Janet Renshaw, Chapter 19 [540LIS] 2. Chemistry for Advanced Level. Peter Cann, Peter Hughes, Chapter 23 [540CAN]