Alkanes Flashcards
what are alkanes?
-naturally occurring hydrocarbons formed from crude oil
what are alkanes formed by?
The conversion of animal and vegetable matter, which existed under pressure for millions of years
why are alkanes saturated compounds?
since they contain single c-c and c-h covalent bonds only
what is the shape of an Alkane?
-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
What are the bond angles of alkanes?
109°
What are all bonds between carbon atoms or between a carbon and hydrogen atom?
-Sigma bonds
what are all sigma bonds able to do?
freely rotate
what are sigma bonds formed by?
Sigma bonds are formed by the direct overlap of orbitals directly between the bonding atoms
what does alkanes being saturated hydrocarbons mean?
They have no double or triple bonds
What are the three distinctive types of alkanes?
-straight chained alkanes
-branched chained alkanes
-ring or cycloalkanes
What is an example of a straight chained alkane?
Butane
What is an example of a branched chained alkane?
2-methylpropane
What is an example of a ring or cyclo alkane?
cyclobutane
What type of forces are between the many (successive) molecules of an alkanes?
Induced dipole dipole interactions
What happens to boiling point as the length of the carbon chain increases?
-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
why are the attractive forces between branched-chained molecules less than the forces between straight-chained molecules?
-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
why is the boiling point, melting point and density of the straight chain alkane higher corresponding branched isomer?
more energy is needed to break the induced dipole dipole forces in a straight chained alkane
Why do all show little reaction?
-they have strong single bonds (both C-H and C-C have high bond enthalpies )
-They have no polar bonds
when is a polar bond formed?
when two elements of differing electronegativities are bonded together
Why is the bond between carbon and hydrogen only very slightly polar?
-carbon and hydrogen have similar electronegativities
-Consequently, common chemical reagents do not readily attack alkanes
when do alkanes burn completely to produce carbon dioxide and water?
in a plentiful supply of oxygen
What is oxidation of alkanes the basis for?
-their use as fuels
what is natural gas used for? (burning of alkanes)
-heating purposes
what is diesel used for? (alkanes)
power
What happens to alkanes in a limited supply of oxygen?
alkanes burn to form carbon monoxide and water
-CH4 (g) + 1/2 O2 (g) —> CO (g) 2H2O (g)
what indicates in incomplete combustion?
-The black substance (soot) produced by a smoky flame (which is largely carbon)
What are the dangerous arising from carbon monoxide?
-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
when do alkanes react with halogens?
(halogenation)
in the presence of ultraviolet light, they undergo a series of substitution reactions
Why is hydrogenation an example of a photo chemical reaction?
It’s only occurs in the presence of light
what must happen to the covalent bonds for halogenation to take place?
-they have to be broken homiletic to form free radicals
-In these reactions, a hydrogen atom alkane is substituted by a halogen atom
what happens when an alkane and halogen react?
Haloalkane and H (X)
What is the mechanism for free radical substitution of alkanes?
-initiation
-Propagation
-Termination
What is a mechanism?
A sequence of steps leading to the formation of a product
What are the conditions for radical substitution? (of chlorine and methane)
Reagents- Methane and Chlorine
-Conditions- Ultra Violet Light
