12.2 Chemical reactions of the alkanes Flashcards

1
Q

Do alkanes react with common reagents

A

They do not react with most

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2
Q

What are the reasons for the lack of reactivity of alkanes

A

Their C-C and C-H σ- bonds are strong
C-C bonds are non-polar
The electronegativity of carbon and hydrogen is so similar that the C-H bond can be considered to be non-polar

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3
Q

Despite their low reactivity, all alkenes react with a plentiful supply of ______ to produce ______ ______ and ______. This reaction is called combustion

A

Oxygen
Carbon dioxide
Water

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4
Q

Why are alkanes used as fuels

A

They are readily available, easy to transport and burn in a plentiful supply of oxygen without releasing toxic products

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5
Q

Methane info

A

Greenhouse gas with global warming potential of 22
Vast quantity stored in ice-saturated arctic tundra of Siberia and N. Canada
Formed by decomposition of organic material

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6
Q

What do alkanes burn completely to produce in a plentiful supply of oxygen

A

Carbon dioxide and water

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7
Q

What happens if oxygen is limited during the combustion of alkanes

A

The hydrogen atoms in the alkane are always oxidised to water, but combustion of the carbon may be incomplete, forming carbon monoxide, or carbon itself as soot

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8
Q

Carbon monoxide info

A

Colourless, odourless and highly toxic gas.
Combines irreversibly with haemoglobin in red blood cells to form cherry-pink compound called carboxyhaemoglobin which prevents haemoglobin from transporting oxygen around body

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9
Q

When do alkanes react with halogens

A

In the presence of sunlight

High energy UV radiation present in sunlight provides initial energy for a reaction to take place

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10
Q

What is an example of radical substitution

A

The mechanism for the bromination of methane

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11
Q

Three stages of the mechanism for the bromination of methane (radical substitution)

A

Initiation, propagation and termination

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12
Q

What happens in the initiation stage (radical substitution)

A

STEP 1
Reaction is started when covalent bond in bromine molecule is broken by homolytic fission
Each bromine atom takes one electron from the pair, forming two highly reactive bromine radicals
Energy for this bond is provided by UV radiation
Br-Br->Br•+•Br

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13
Q

What is a radical

A

A very reactive species with an unpaired electron

Shown with a single dot to represent the electron

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14
Q

What happens in the propagation stage (radical substitution)

A

STEP 2
Reation propagates through two steps, a chain reaction
CH4+Br•->•CH3+HBr
•CH3+Br2->CH3Br+Br•
In first, bromine radical Br• reacts with C-H bond in methane, forming methyl radical, •CH3 and molecyle of HBr
In second, each methyl radical reacts with another bromine forming bromomethane with new bromine radical
New bromine radical reacts with another CH4, continues as cycle
Terminated when radicals collide

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15
Q

What happens in the termination stage (radical substitution)

A

STEP 3
Two radicals collide, forming molecule with all electrons paired, Number of possible termination steps with different radicals in reaction mixture
Br•+•Br->Br2
•CH3+•CH3->C2H6
•CH3+VBr->CH3Br
When two radicals collide and react, both radicals are removed from the reaction mixture, stopping the reaction

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16
Q

Limitations of radical substitution in organic synthesis

A

Another bromine radical may collide with a bromomethane molecule, substituting a further hydrogen atom to form dibromomethane, further substitutions may continue until all hydrogen atoms have been substituted resulting in a mixture of molecules
For methane, only one monobromo compound is possible; if the carbon chain is longer, we will get a mixture of monosubstituted isomers