Bài giảng hoá học hữu cơ alkanes

 Alkanes are hydrocarbons, they contain only carbon and hydrogen..  If you continue the successive substitution hydrocarbons, each different from one organic compounds in which the mem

 Alkanes are hydrocarbons, they contain

only carbon and hydrogen

integer

compounds, meaning “fat”

atoms connected by sigma bonds

 The simplest molecules is methane, CH4 -

the atmospheres of the outer gas planets

methane, 1.5 ppm ethane)

decomposition of organic substances

(wood, lignite and mineral coal)

for alkanes are natural gas and oil

Homologous series

 What is a homologous series?

 A homologous series is a series of organic

compounds with a similar general formula,

possessing similar chemical properties due to the presence of the same functional group, and

shows a gradation in physical properties as a

result of increase in molecular size and mass.

Homologous series

Homologous series

will be obtained

will be obtained

 If you continue the successive substitution

hydrocarbons, each different from one

organic compounds in which the members differ by a constant atomic mass of 14

Isomerism

 Isomers are compounds that have the same numbers and kinds of atoms but differ in the way the atom arranged

Isomerism

 Isomeric Alkanes

can be arranged in a multiple number of

ways, forming different structural isomers (constitutional isomers)

attachment of atoms

 The carbon atoms in alkanes can be

(n for "normal")

branched at one or more points

CH 3 - CH 3 - CH 2 - CH 3 CH 3 - CH - CH 3

CH 3

 The number of possible isomers increases rapidly with the number of carbon atoms.

artifacts They’ve been retained due to

familiar usage in industry

chain are called:

methane, ethane, propane, butane

 Alkanes with five or more carbon atoms are

appropriate Greek-numeric prefix

 The alkane names form the basic for

naming all other organic compounds, so at least the first ten should be memorized

 For the hydrocarbons without branch, use the

 For the hydrocarbons with two methyl

hydrocarbon

iso -pentane

 For those containing three -CH3 groups at

+ the name of the hydrocarbon

neo –hexane

 Primary carbon (1°) - linked to one other

Alkyl groups

 If one hydrogen atom is removed from an

alkane, the partial structure that remains is called an alkyl group.

 They are named by the replacement of the

ending “-ane ane” from the hydrocarbon’s name with the ending “-yl”.

 Alkyl groups are not stable compounds

themselves, they simply parts of larger

compounds.

 Removal of a hydrogen atom from the end carbon of an n-alkane gives an n-alkyl

group

 Branched alkyl groups are generated by removing a hydrogen from an internal carbon

 n-propyl and isopropyl groups

 Four 4-carbon alkyl groups:

The prefixes sec- (for

secondary) and tert- (for

tertiary) used for the C4

alkyl groups refer to the

number of other carbon

atoms attached to the

branching carbon atom

IUPAC nomenclature of

Organic compounds

IUPAC is the abbreviation of International

Union of Pure and Applied Chemistry

IUPAC naming rules

in the molecules and use the name of that

CH 3 CH 2 CH 2 CH CH 3

Name as a substituted hexan

 If two chains of equal length are

present, choose the one with the larger

number of branch points as the parent:

 Step 2 Number the atom in the main chain

branch point, number each carbon atom in the parent chain:

 The first branch occurs at C3 in the proper system of numbering, but at C4 in the improper system.

4 5

3

 Step 3 Identify and number the substituents

group) according to its point of attachment

on the parent chain

CH 2 CH 3

CH 3 CH CH 2 CH 2 CH - CH CH 2 CH 3

CH 2 CH 3

1 2

3 4

5 6 7

 If there are two substituents on the same

 Step 4 Write the name as a single word

 Use hyphens to separate the various

 If two or more identical side chains are

tetra-, and so forth

 Correction the second comp.: 4-ethyl-3-methylheptane.

4 5

6

4 5

 The following trivial names are retained in the IUPAC system:

isobutane for 2-methylpropane,

 Sometimes common names for substituents can be incorporated:

compounds:

following IUPAC names:

(a) 3,4-Dimethylnonane

(b) 3-Ethyl-4,4-dimethylheptane

(c) 2,2-Dimethyl-4-propyloctane

Solution

Conformation of ethane

able to rotate freely, with

respect to one another,

about C-C axis

 Orbital overlap in the C-C bond is exactly

the same for all geometric arrangements of the hydrogens

result from rotation around a single bond

conformation and staggered conformation

The staggered conformation is 12.6

kJ/mol lower in energy (the more stable) than the eclipsed conformation (the least stable)

 Chemists represent different conformations

in two ways

 Sawhorse or Dash-Wedge structures

 Newman projections view the C-C bond end-on and represent the two carbon atoms

by a circle

H

H H

 Bond attached to the front carbon are represented by lines to the center of the circle.

represented by lines to the edge of the circle

 Spatial modeling of Eclipsed (left) and

Staggered (right)

Conformation of Butane

longer equivalent and represent two distinct

right-most)

eclipsed-conformation

gauche-conformation

video

Laboratory preparation

Hydrogenation of unsaturated hydrocarbons

in the presence of metal catalyst such as

Ni, Pd and Pt to produce alkanes:

Ni Ni

From halogenated derivates

 The Wurtz reaction between alkyl halides and sodium leads to the formation of

symmetrical alkanes:

R-X + R-X + 2Na → 2NaX + R-R

X can be: Cl, Br, I

 Alkyl halides are converted into

alkylmagnesiumhalides and then reduced to alkanes

place under heating with CaO (+NaOH):

Physical properties

At home,

read the physical properties of alkanes !

Physical properties

physical properties of alkanes

intermolecular attraction in alkanes is van der Waals forces (but so weak)

Diagram of Bp, Mp and density

 As an alkane molecule approaches another, electron repulsion causes a subtle (unclear) redistribution of electron density leading to temporary bond polarization in the

 A straight-chain alkane will have a boiling point higher than a branched-chain alkane due to

between adjacent molecules

 Boiling point of Isobutane: -12°C

n-Butane: 0°C

Branching reduces surface area:

Chemical reactions

break unless alkanes are heated to very high temperature

420 kJ/mol and of C-C: 376 kJ/mol

 Alkanes are generally unaffected by most bases and acids

polarized (nearly the same electronegavity)

breakage producing free radicals

which a C-H bond is broken and a new

 In this reaction a halogen atom replaces one

or more of hydrogen atoms of the alkane

produces a mixture of:

 The reactivity of hydrogen atoms can be

ranked in the order:

mixture of isomeric monochloro products as well as more highly halogenate compounds

 Chlorination of propane at 25 ºC gives

alkanes than chlorine

attack when it does react

 The reaction of bromine with isobutane

produces almost one product.

Radical mechanism of Halogenation

 Free radical substitution (SR) occurs in 3 steps.

 Initiation the halogen radicals form by

homolysis

initiators (peroxides) is required

.

 Chain reaction or Propagation.

from the alkane to give an alkyl radical

 This reacts further

 Chain terminationChain termination step where the radicals

Nitration reaction

phase at high temperatures produces to

the most important source of heat energy

CnH 2n+2 + (1.5n+0.5)O 2 → (n+1)H 2 O + nCO 2

 Incomplete oxidation in the presence of a catalyst leads to various products.

R-CH 3 + O 2 → R-CH2-OH (alcohol) → R-CH=O (aldehyde) → RCOOH (acid)

Thermic decomposition

smaller ones

homolytic mechanism with formation of free-radicals

 Acid catalysts are used for cracking (such

as silica-alumina and zeolites)

bonds yielding a carbocation and the very unstable hydride anion

 The products of thermic decomposition are

chemical industry

where n = m + p and p >= 2