unit 1.6 alkane

7/28/2019 Unit 1.6 Alkane

1/49

Organic

Chemistry

Alkanes


2/49

Learning Outcomes

state the general formula of alkanes and understand that they are

saturated hydrocarbons which contain single bonds only

explain the existence of structural isomers using alkanes (up to

C5) as examples

know that alkanes are used as fuels and obtained from thefractional distillation, cracking and reformation of crude oil

discuss the reasons for developing alternative fuels in terms of

sustainability and reducing emissions, including the emission of

CO2 and its relationship to climate change describe the reactions of alkanes in terms of combustion and

substitution by chlorine showing the mechanism of free radical

substitution in terms of initiation, propagation and termination,

and using curly half-arrows in the mechanism to show the


3/49

1. The general formula foropen chain alkanes is CnH2n+2.Alkanes are saturated hydrocarbons; only CCsingle bonds

2. Click 1, 2, 3, and 4 one at a time to view the molecular,

structural formulas, the IUPAC name and 3-D shapes of thefirst 4 members of the alkane series


4/49

1. IUPAC names of alkanes have 2 parts. The prefix indicatesthe number of C atoms in the chain and the suffixane

shows that it is an alkane. Prefix like met, et, prop, but, hex

2. Click +CH2 to obtain other members of the series.


5/49

1. If a H atom is removed, the structure that remains is called analkyl group. Refer to table 8.2 pg 190 of text

2. Click to remove red H atom. Note name of alkyl formed

3. Name of alkyl groups removeane replace with yl

4.CH CH CH CH is called buth l


6/49

1.Click on alkyl groups to replace H atom in red

2. A branched alkane is obtained. Note the name.


7/49

1. In order to name branched-chain alkanes follow the steps

given above.


8/49


9/49


10/49

Try naming alkanes by naming the alkanes given on

slides 46-54 of Introductory Organic Chemistry


11/49


12/49

ALKANES

Homologous series of saturatedaliphatic hydrocarbon of general

formula CnH2n+2.

names of alkanes end with ane

We only focus on aliphatic alkane

here

(A) INTRODUCTION


13/49

-------------------------------------------------------------------------------------

--------n Molecular name Structural

formula formula

------------------------------------------------------------------------

--------1 CH4 methane CH4

2 C2H6 ethane CH3CH3

3 C3H8 propane CH3CH2CH34 C4H10 butane CH3(CH2)2CH3

5 C5H12 pentane CH3(CH2)3CH3

6 C6H14 hexane CH3(CH2)4CH3

(A) INTRODUCTION


14/49

At room temperature and atmosphericpressure, first 4 alkanes (C1 - C4) are

gases; next 13 alkanes (C5-C17) are

liquids ; C18 or more are solids.

They are insoluble in water

(B) PHYSICAL PROPERTIES


15/49

(C) Where are alkanes obtained from?

Finite Source: Fossil Fuels (Crude oil)

Takes millions of years to form, conditions

are no longer suitable for formation

Cannot be replenished and replaced


16/49

(D) Using Crude oil

Fractional distillation

Different length carbon chain have different

boiling point.

Different sized molecules also have

different uses.

Lighter fractions are in greater demand

catalytic cracking or reforming of heavier

fractions to form lighter ones or more

branched ones


17/49

1. Petroleum or crude oil is a complex mixture of hydrocarbons

mainly alkanes. Petroleum can be separated into fractions by

fractional distillation.

2. Click on each fraction above to view its uses.


18/49

Petroleum (crude oil) is an important source of energy and

chemicals. Fraction petroleum gas. Gas oil is also diesel oil

Naphta also used in cracking to produce high grade petrol.


19/49

Unreactive towards metals, mineralacids, oxidising agents.

Reason: both C-C and C-H bonds involve

a very even sharing of electrons notpolar, so no attraction to other polar or

ionic species.

Reactions involve free radicals

(E) CHEMICAL PROPERTIES OF ALKANE


20/49

Types of bond fission(breaking of covalent bonds)

a) Homolytic Fission

- Two shared electrons in the covalent bond aresplit equally between the two atoms.

(F) BREAKING BONDS


21/49

Eg.

Product is free radical (atoms or groups of atoms with

a single unpaired electron).


22/49

b) Heterolytic Fission

- Two shared electrons in the bond are split

unequally between the two atoms. One of the

atoms keeps both electrons and hence acquiresa negative charge. The other atom is deficient

in one electron and hence has a positive

charge.


23/49

Movement of electrons is depicted by curved arrow

( ) which denotes the shift of a pair of electronsfrom the tail to the head of the arrow. The shift may

be from a bond onto an atom or from an atom into a

bond.

Eg.

R C X

H

H

R C

H

H

+ + X_


24/49

Eg.

R C

H

H

+_

R C

H

H

+Cl Cl


25/49

1. Alkanes are hydrocarbons that burn readily in air or oxygen

2. Click on both containers to ignite CH4 gas.

3. In excess oxygen, CO2, H2O and heat is produced

4. In limited oxygen, besides CO2, unburnt C, CO and heat is

generated


26/49

(a) Limited supply of oxygen

-- incomplete combustion to produce

CO (or C) and H2Oeg.

CH4 O2 C 2H2O

2CH4 3O2 2CO 4H2O

(F) REACTIONS: COMBUSTION


27/49

(b) Unlimited supply of oxygen

-- complete combustion to produce CO2

and H2O

eg.

CH4 2O2 CO2 2H2O



28/49

* methane (natural gas or cooking gas) burnswith a clean blue flame tinged with a little

yellow. Very little soot (carbon) is formed.

Burning of higher alkanes gives rise to sooty

flames.

Note:

* reaction is exothermic* ease of burning accounts for the use of manyalkanes as fuels



29/49

(G) REACTIONS: CRACKING

Thermal decomposition in the absence of

air.

C-C bonds broken to form smallermolecules

E.G

C2H6 (g) CH2=CH2(g) + H2(g)


30/49

Cracking of Crude oil (Petroleum)

The process in which the carbon carbon bonds in long chain

alkanes (large hydrcarbon molecules) are broken producing

smaller molecules of both alkanes, alkenes and evenhydrogen

CRACKING

Thermal cracking

Conditions: high

temperatures

(8000C 9000C)

high pressures

Catalytic Cracking

Conditions:(4500 5500C)

Finely divided silica-alumina catalyst,

slight pressures


31/49

Equations:

1. CH3(CH2)14 CH3 CH3(CH2)7 CH=CH2

+ CH3CH2CH2CH2CH2CH3

2. CH3(CH2)8CH3 CH3CH=CH2 + CH3(CH2)5CH3

3. C4H10 C4H8 + H2

C2H6 + C2H4

CH4 + C3H6

Uses: 1) produce alkenes starting materials to makeplastic

2) produce high grade petrol.


32/49

Characteristics of free-radical mechanism:

-- uv light or high temperature needed to

generate free radical

-- chain reaction involving 3 basic steps :-

initiation, propagation and termination.

(H) REACTIONS: HALOGENATION

FREE-RADICAL SUBSTITUTION MECHANISM


33/49

Example: Chlorination of methane via free

radical substitution mechanism

Initiation: Heat/light provides energy for

homolytic cleavage of Cl-Cl bond to

generate free radical.

uv or

C2Cl

free radicalCl Cl

200_400o



34/49

Here, each step consumes a radical and

generates another. Together, these two

propagation steps consume one CH4 molecule

and one Cl2 molecule and produce one HCl

molecule and one CH3Cl molecule, giving the net

reaction.


Propagation


35/49

Cl CH4 CH3 HCl

CH3 Cl2 Cl CH3Cl



36/49

Termination:

Here, free radicals are consumed but not

generated.

Cl Cl Cl2

CH3 CH3 C2H6

Cl CH3 CH3Cl



37/49

OVERALL:

CH4 Cl2 CH3Cl HCluv light

or heat



38/49

Note:

a) a single free radical (once formed) can produce

a large number of molecules of product by chainreaction sequence.

b) reaction does not stop at this step

(monosubstitution). More than one hydrogenatoms in alkane can be replaced by chlorine with

elimination of HCl.



39/49

1. The substitution reaction involves the replacement of 1 or more H atomswith Cl or Br aroms.

2. Click CH4 molecule to substitute hydrogen with chlorine

3. The substitution reaction involves a free radical. The reaction is called a

free radical substitution reaction


40/49

CH4 Cl2 CH3Cl HCl(monosubstitution)

CH3Cl Cl2 CH2Cl2 HCl

(disubstitution)

CH2Cl2 Cl2 CHCl3 HCl(trisubstitution)



41/49

CHCl3 Cl2 CCl4 HCl

(complete substitution)Hence reaction is not clean meaning that it

does not give a single organic product.



42/49

c) Monosubstituted alkane, CH3Cl,

predominates if excess of alkane is used.

Fully substituted product, CCl4,predominates with an excess of Cl2.



43/49

d) Unlike methane, ethane and higher alkanes

give rise to isomeric halogen derivatives when

more than one hydrogen atom is substituted.

eg. there are 2 dichloroethanes, CH2ClCH2Cl and

CH3CHCl2. This is typical of reactions of alkanes:

they tend to be indiscriminate because there is

little difference between the strengths of thedifferently situated C-H bonds.



44/49

Please Read Text pg 120-123

Learning outcome:

discuss the reasons for developing alternative fuels in

terms of sustainability and reducing emissions,

including the emission of CO2 and its relationship to

climate change

(I) HSW: Ethical Issues


45/49

AIR POLLUTION FROM MOTOR VEHICLE EMISSIONS

Compounds in car

exhausts

% Compounds in car

exhausts

%

Water

Carbon dioxide

Carbon monoxide

Oxygen

9

8

4-6

4

Hydrogen

Unburnt hydrocarbons

Oxides of nitrogen

Lead(II) bromide vapour

2

0.2

0.3

10-3

Pollutants in exhaust fumes

a) Carbon monoxidehighly toxic and deadly gas. It bonds

with haemoglobin more strongly than O2 and once combinedwith haemoglobin cannot be displaced by O2, hence

prevents haemoglobin from carrying O2 to cell tissues.

b) Carbon dioxide causes green house effect which leads to

global warming.


46/49

c)Oxides of nitrogen:

Nitrogen dioxide gas is corrosive and irritating to respiratorytract; gives rise to respiratory problems like asthma and hay

fever

Photochemical smog formed from photochemical reactions

between unburnt hydrocarbons, oxygen and oxides ofnitrogen. Photochemical smog is a grave air pollutantiritating to eyes and respiratory system. Main components in

smog are ozone and PAN

Causes formation of acid rain. This rain contains HNO3 andHNO2.Acid rain threatens fish and plankton life in streams,

rivers, lakes and to tree life.


47/49

d) Unburnt hydrocarbons

can be very toxic and even carcinogenic (benzene); also

cause liver damage; a component of photochemical smog a dangerous air pollutant.

e) Lead(II) bromide

This is released into the atmosphere when leaded petrolis burnt in air. Atmospheric air causes high levels of lead in

bodies of people living in towns and cities where there is

much motor emmisions.

Lead compounds are highly toxic when inhaled.Accumulation in the body causes brain damage as well asdamage to central nervous system

f) Sulphur dioxide: combustion of petrol containing

sulphur. Dissolves in rain water acid rain.


48/49

How to reduce knocking and improve performance ofcar engines?

1. Use branched chain hydrocarbons achieved through

cracking.

2. Add anti-knocking agent tetraethyllead Pb(C2H5)4. . 1,2

dibromoethane is also added to prevent accumulation of Pbin the engine. However volatile PbBr2 is emitted through the

exhaust into the atmosphere cause atmospheric pollution.

3. Use unleaded petrol which has a high octane rating above

90 by adding alcohols or methyl-tertiary-buthyl ether.

(CH3)3COCH3


49/49

How to reduce car exhaust emissions?

Using a catalytic converter

1. three-way catalytic converter consisting of a ceramic

honeycomb structure lined with platinum, paladium andrhodium catalysts

2. Capable of removing carbon monoxide, oxides of nitrogenand unburnt hydrocarbons

3. CO and unburnt CxHy are catalytically oxidised as follows

2CO(g) + O2(g) 2CO2(g)CxHy + (x + y/4) O2 xCO2 + y/2 H2O

4. Oxides of nitrogen are catalytically reduced by CO

2NO + 2CO N2 + 2CO2

unit 1.6 alkane

Documents