Alkanes Flashcards

1
Q

what are alkanes?

A

-naturally occurring hydrocarbons formed from crude oil

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2
Q

what are alkanes formed by?

A

The conversion of animal and vegetable matter, which existed under pressure for millions of years

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3
Q

why are alkanes saturated compounds?

A

since they contain single c-c and c-h covalent bonds only

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4
Q

what is the shape of an Alkane?

A

-Each carbon bond makes four bonds
-Around each atom, there are 4 bonding pairs of electrons which repel.
-Thus each carbon atom is at the centre of a tetrahedral structure, with either carbon or hydrogen atoms at the apices of the tetrahedron

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5
Q

What are the bond angles of alkanes?

A

109°

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6
Q

What are all bonds between carbon atoms or between a carbon and hydrogen atom?

A

-Sigma bonds

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7
Q

what are all sigma bonds able to do?

A

freely rotate

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8
Q

what are sigma bonds formed by?

A

Sigma bonds are formed by the direct overlap of orbitals directly between the bonding atoms

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9
Q

what does alkanes being saturated hydrocarbons mean?

A

They have no double or triple bonds

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10
Q

What are the three distinctive types of alkanes?

A

-straight chained alkanes
-branched chained alkanes
-ring or cycloalkanes

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11
Q

What is an example of a straight chained alkane?

A

Butane

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12
Q

What is an example of a branched chained alkane?

A

2-methylpropane

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13
Q

What is an example of a ring or cyclo alkane?

A

cyclobutane

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14
Q

What type of forces are between the many (successive) molecules of an alkanes?

A

Induced dipole dipole interactions

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15
Q

What happens to boiling point as the length of the carbon chain increases?

A

-Boiling point increases
-Larger molecules have greater surface contact and therefore greater induced dipole-dipole interactions
-more energy is required to overcome the greater induced dipole-dipole forces and separate the molecules so the boiling point is higher

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16
Q

why are the attractive forces between branched-chained molecules less than the forces between straight-chained molecules?

A

-because the branched chained molecules cannot get as close together as the straight chain ones, so the induced dipole dipole forces are weaker and easier to break
-The surface contact of branched-chaine isomers is less than that between straight-chain isomers

17
Q

why is the boiling point, melting point and density of the straight chain alkane higher corresponding branched isomer?

A

more energy is needed to break the induced dipole dipole forces in a straight chained alkane

18
Q

Why do all show little reaction?

A

-they have strong single bonds (both C-H and C-C have high bond enthalpies )
-They have no polar bonds

19
Q

when is a polar bond formed?

A

when two elements of differing electronegativities are bonded together

20
Q

Why is the bond between carbon and hydrogen only very slightly polar?

A

-carbon and hydrogen have similar electronegativities
-Consequently, common chemical reagents do not readily attack alkanes

21
Q

when do alkanes burn completely to produce carbon dioxide and water?

A

in a plentiful supply of oxygen

22
Q

What is oxidation of alkenes the basis for?

A

-their use as fuels

23
Q

what is natural gas used for? (burning of alkanes)

A

-heating purposes

24
Q

what is diesel used for? (alkanes)

A

power

25
Q

What happens to alkanes in a limited supply of oxygen?

A

alkanes burn to form carbon monoxide and water
-CH4 (g) + 1/2 O2 (g) —> CO (g) 2H2O (g)

26
Q

what indicates in incomplete combustion?

A

-The black substance (soot) produced by a smoky flame (which is largely carbon)

27
Q

What are the dangerous arising from carbon monoxide?

A

-O2 carried around body in rbc’s by haemoglobin
-The bonding between O2 and Hb is quite weak to allow transfer for cell respiration
-Bonding between O2 and CO is stronger and irreversible so O2 is replaced by CO and blocks normal cell respiration
-The consequences are reduced blood oxygen concentration leading to unconsciousness and eventually death

28
Q

when do alkanes react with halogens?
(halogenation)

A

in the presence of ultraviolet light, they undergo a series of substitution reactions

29
Q

Why is hydrogenation an example of a photo chemical reaction?

A

It’s only occurs in the presence of light

30
Q

what must happen to the covalent bonds for halogenation to take place?

A

-they have to be broken homiletic to form free radicals
-In these reactions, a hydrogen atom alkane is substituted by a halogen atom

31
Q

what happens when an alkane and halogen react?

A

Haloalkane and H (X)

32
Q

What is the mechanism for free radical substitution of alkanes?

A

-initiation
-Propagation
-Termination

33
Q

What is a mechanism?

A

A sequence of steps leading to the formation of a product

34
Q

What are the conditions for radical substitution? (of chlorine and methane)

A

Reagents- Methane and Chlorine
-Conditions- Ultra Violet Light