chap 3_Alkanes and Cycloalkanes

8/7/2019 chap 3_Alkanes and Cycloalkanes

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CHAPTER 3

ALKANES

ALKANES AND CYCLOALKANES


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Course Outcome Ability to explain the relationship between the structure,

physical and chemical properties of the different bonds andfunctional groups in organic compounds (CO2)

Course Learning OutcomeThe student should be able to: -

y Name alkanes.

y Explain aliphatic properties.

y Predict, draw and name the products of functional group reactions.

y Draw the mechanistic pathway.


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Fossil Fuels:

Many alkanes occur in

nature, primarily in

natural gas and

petroleum.

Natural gas is com-posed

largely of methane, with

lesser amounts of ethane,

propane and butane.

Petroleum is a complex mixture of compounds, most of which are hydrocarbons

containing one to forty carbon atoms. Distilling crude petroleum (called refining),

separates it into usable fractions that differ in boiling point.

gasoline: C5H12C12H26kerosene: C12H26C16H34

diesel fuel: C15H32C18H38


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Sources of Alkanes: Petroleum and Natural Gas

.

Petroleum is the source of alkanes. It is a complex mixture of

mostly alkanes and aromatic hydrocarbons with smaller amounts

of oxygen-, nitrogen-, and sulfur-containing compounds

Natural gas is a gaseous mixture of hydrocarbons recovered from

natural sources. It is mostly methane (CH4, BP -162oC) with small

amounts of ethane (C2H6, BP -88oC) and propane (C3H8, BP -42

o).

Petroleum Refining

Liquid petroleum and natural gas are usually separated at the

wellhead and shipped independently to processing (refining) plants.

The liquid petroleum (crude) is separated by distillation according to

the volatility (BPs) of the different size hydrocarbons. The fractions

collected by refining are still mixtures of hydrocarbons that have

commercial value.


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Refining crude petroleum into usable fuel and other petroleum products.(a) An oil refinery. At an oil refinery, crude petroleum is separated into

fractions of similar boiling point by the process of distillation.

(b) Schematic of a refinery tower. As crude petroleum is heated, the lower-

boiling, more volatile components distill first, followed by fractions of

progressively higher boiling point.


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Catalytic eforming

Alkanes are transformed into cycloalkanes and aromatic

ydrocar ons y catalytic reforming.

eptane

silica-al mina

catalyst, 500oC

20 atm H2

+ 4H2CH3CH2CH2CH2CH2CH2CH3

CH3

e aromatic ydrocar ons prod ced y catalytic reforming

are sed as additives in gasoline and as starting materials for

t e petroc emical ind stry. Prod ction of t ese aromatics is in

t e illions of po nds per year in t e United tates.


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Physical Properties of Alkanes


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Nomenclature Recall that alkanes are aliphatic hydrocarbons having CC and CH W bonds.

They can be categorized as acyclic or cyclic.

Acyclic alkanes have the molecular formula CnH2n+2 (where n = an integer) and

contain only linear and branched chains of carbon atoms. They are also calledsaturated hydrocarbons because they have the maximum number ofhydrogen atoms per carbon.

Cycloalkanes contain carbons joined in one or more rings. Because theirgeneral formula is CnH2n, they have two fewer H atoms than an acyclic alkanewith the same number of carbons


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All C atoms in an alkane are surrounded by four groups, making them sp3hybridized and tetrahedral, and all bond angles are 109.50.

The 3-D representations and ball-and-stick models for these alkanesindicate the tetrahedral geometry around each C atom. In contrast, theLewis structures are not meant to imply any 3-D arrangement. Additionally,in propane and higher molecular weight alkanes, the carbon skeleton canbe drawn in a variety of ways and still represent the same molecule.


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The three-carbon alkane CH3CH2CH3, called propane, has a molecular

formula C3H8. Note in the 3-D drawing that each C atom has two bonds

in the plane (solid lines), one bond in front (on a wedge) and one bond

behind the plane (on a dashed line).


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In a Lewis structure, the bends in a carbon chain dont matter.

There are two different ways to arrange four carbons, giving two compounds

with molecular formula C4H10, named butane and isobutane. Butane and isobutane are isomers, two different compounds with the same

molecular formula. Specifically, they are constitutional or structural isomers.

Constitutional isomers differ in the way the atoms are connected to each other.


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Carbon atoms in alkanes and other organic compounds are classified by thenumber of other carbons directly bonded to them.


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Hydrogen atoms are classified as primary (10), secondary (20), or tertiary (30)depending on the type of carbon atom to which they are bonded

The maximum number of possible constitutional isomers increases dramaticallyas the number of carbon atoms in the alkane increases. For example, there are75 possible isomers for an alkane having 10 carbon atoms, but366,319 possibleisomers for one having 20 carbons.


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Cycloalkanes

Cycloalkanes have molecular formula CnH2n and contain carbon atoms

arranged in a ring. Simple cycloalkanes are named by adding the prefix

cyclo- to the name of the acyclic alkane having the same number of

carbons.


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Nomenclature

The name of every organic molecule has 3 parts:

1. The parent name indicates the number of carbons in the longest

continuous chain.

2. The suffix indicates what functional group is present.

3. The prefix tells us the identity, location, and number of substituents

attached to the carbon chain.


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Carbon substituents bonded to a long carbon chain are called alkyl

groups.

An alkyl group is formed by removing one H atom from an alkane.

To name an alkyl group, change the ane ending of the parent alkane to

yl. Thus, methane (CH4) becomes methyl (CH3-) and ethane (CH3CH3)

becomes ethyl (CH3CH2-).

Naming three- or four-carbon alkyl groups is more complicated becausethe parent hydrocarbons have more than one type of hydrogen atom.


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For example, propane has both 10 and 20 H atoms, andremoval of each of these H atoms forms a different alkylgroup with a different name, propyl or isopropyl.


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1. Find the parent carbon chain and add the suffix.

Note that it does not matter if the chain is straight or it bends.

IUPAC systematic Nomenclature - Alkanes


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Also note that if there are two chains of equal length, pick the chain with

more substituents. In the following example, two different chains in the

same alkane have seven C atoms. We circle the longest continuous chain

as shown in the diagram on the left, since this results in the greaternumber of substituents.


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2. Number the atoms in the carbon chain to give the first substituent the

lowest number.


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If the first substituent is the same distance from both ends, number the

chain to give the second substituent the lower number.


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When numbering a carbon chain results in the same numbers from either

end of the chain, assign the lower number alphabetically to the first

substituent.


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3. Name and number the substituents.

Name the substituents as alkyl groups.

Every carbon belongs to either the longest chain or a substituent, not

both.

Each substituent needs its own number

If two or more identical substituents are bonded to the longest chain, use

prefixes to indicate how many: di- for two groups, tri- for three groups,

tetra- for four groups, and so forth.


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4. Combine substituent names and numbers + parent and

suffix.

Precede the name of the parent by the names of the substituents.

Alphabetize the names of the substituents, ignoring all prefixes except

iso, as in isopropyl and isobutyl.

Precede the name of each substituent by the number that indicates itslocation.

Separate numbers by commas and separate numbers from letters by

hyphens. The name of an alkane is a single word, with no spaces after

hyphens and commas.


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2. Name and number the substituents. No number is needed to indicate

the location of a single substituent.

For rings with more than one substituent, begin numbering at one

substituent and proceed around the ring to give the second substituent the

lowest number.


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With two different substituents, number the ring to assign the lower

number to the substituents alphabetically.

Note the special case of an alkane composed of both a ring and a long

chain. If the number of carbons in the ring is greater than or equal to the

number of carbons in the longest chain, the compound is named as a

cycloalkane.


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Some organic compounds are identified using common names that do not

follow the IUPAC system of nomenclature. Many of these names were

given long ago before the IUPAC system was adopted, and are still widely

used. Additionally, some names are descriptive of shape and structure, likethose below:

NomenclatureCommon Names


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Hydrogenation of alkenes and alkynes

CnH2n C nH2n+2

H2

Pt, Pd r i

lk ne alkane

H2/Ni

C2H5OH25

o, 50at

(CH3)3CH

Synthesis of alkanes and cycloalkanes

Pt

+ 2H2

+ H2Pd


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Reduction of alkyl halidesi. Hydrolysis with Grignard reagent

R-X + Mg R Mg X

RMgX + HOH R-H + Mg(OH)X

CH3CH2CH2MgBr + H2O CH3CH2CH3 + Mg(OH)Br

ii. Reduction of alkyl halide with metal and acid

(Zn in CH3COOH or HBr)

R-X R-H

CH3CHBrCH2CH3 CH3CH2CH2CH3

Ether(THF)

iii. Reaction with LiAlH4

C9H19CH2-Br C9H19CH3


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Alkylation ofterminal alkynesAn acetylenic hydrogen is weakly acidic:

C C HRNa

NH3

C CR-

Na+ + 1/2H2

a sodiumacetylide

(CH3)2CHC C H

NaNH2

ether(CH

3)2CHC C

-Na

+

+ NH3


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Alkylation ofterminal alkynesThe anion formed will react with a primary halide:

C C-Na

+R + CH3X C CCH3 + NaXR

1. NaNH2

2. CH3Br

H2/Pt


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Oxidation / combustion of Alkanes

Since alkanes are the only family of organic molecules that have

no functional group, they undergo very few reactions.

One reaction that alkanes undergo is combustion.

Combustion is an oxidation-reduction reaction

Recall that oxidation is the loss of electrons and reduction is the

gain of electrons.

To determine if an organic compound undergoes oxidation or

reduction, we concentrate on the carbon atoms of the startingmaterial and the product, and compare the relative number of C

H and CZ bonds, where Z = an element more electronegative

than carbon (usually O, N, or X).


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Oxidation of Alkanes

Oxidation results in an increase in the number of CZ bonds; or

Oxidation results in a decrease in the number of CH bonds.

Reduction results in a decrease in the number of CZ bonds; or

Reduction results in an increase in the number of CH bonds.

Reaction of Alkanes


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Combustion of Alkanes

Alkanes undergo combustionthat is, they burn in the presence of

oxygen to form carbon dioxide and water.

This is an example of oxidation. Every CH and CC bond in the

starting material is converted to a CO bond in the product.


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Reactions of alkanes withhalogens

C H +X2

250- 00o

or RC X+HX

Reacti it :- X2 : 2 > Cl2 > r2 (> I2)

H : 3o

> 2o

>o

> H3C-H

Chlorination - a substitution reaction

CH4+Cl2R

or

(

CH3Cl +HCl


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Polychlorination

CH3Cl + Cl2 CH2Cl2 + HCl

CH2Cl2 + Cl2 CHCl3 + HCl

CHCl3 + Cl2 CCl4 + HCl

dichloromethane

methylene chloride

trichloromethanechloroform

tetrachloromethane

carbon tetrachloride

Iodination and fluorination

iodine does not react while, fluorine reacts very readily

order of halogen reactivity: F2 Cl2 Br2 ( I2)


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A Mechanism

it must explain all experimental facts

the mechanism should be tested by devising appropriate

experiments - mechanistic predictions must be tested in the lab

a detailed, step by step, description of the transformation of reagents

into products


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Mechanism ofthe chlorination of methane

2. Reaction readily occurs, in the absence of light, at

temperatures above 250C.

3. Reaction occurs at room temperature in the presence of light of

a wavelength absorbed by chlorine.

1. No reaction occurs at room temperature in the absence of light.

The experimental facts

5. The presence of even a small quantity of oxygen slows down

the reaction.

4. When the reaction is initiated by light, a large number of

chloromethane molecules are produced for each photon of lightabsorbed by the system.


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Mechanism ofthe halogenation

1. X 2 2X initiation250-400

o

or R

2. X + RH HX + R

3. R + X2 RX + X

propagation

2, 3, 2, 3, 2, 3....etc.


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Chain Reaction

Chain initiation:

Cl-Cl 2Cl

Chain ro a ation:

Cl + CH4

CH3 + Cl2

CH3 +HCl

CH3Cl + Cl

Chain ter ination:

2Cl

2CH3

Cl+

CH

3

Cl2C2H6 (ethane)

CH

3Cl

Refer ( CH10; pg 435)


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The intermediate alkyl radical

The nature of the intermediate free radical determines the product:

CH3CH2CH3X

propane

CH3CH2CH2n-propyl radical

CH3CHCH 3

isopropyl radical

CH3CH3X

CH3CH2

ethane ethyl radical

X2CH3CH2X

haloethane

X2CH3CH2CH2X1-halopropane

X2CH3CHXCH32-halopropane

X2CH3X

halomethaneCH4

X CH3methane methyl radical


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EXERCISE 2

In the presence of light, methane (1 mol) reacts with chlorine (1

mol) to form which product(s)?

A) CH3Cl

B) CH2Cl2

C) CHCl3

D) CCl4

E) All of these


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Lipids Lipids are biomolecules that are soluble in organic solvents and

insoluble in water.

Lipids have varied sizes and shapes, and a diverse number of

functional groups. Lipids are composed of many nonpolar CH and CC bonds, and

have few polar functional groups.

The metabolism of lipids provides energy for our bodies.


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LipidsCholesterol

Cholesterol is a member of the steroid family, a group of lipids having

four rings joined together. Because it has just one polar OH group, it is

insoluble in the aqueous medium of the blood.

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chap 3_Alkanes and Cycloalkanes