12- Alkanes Flashcards
What are alkanes?
-main components of natural gas, crude oil
-amongst the most stable organic compounds
-lack of reactivity has allowed crude oil deposits to remain the the earth for millions of year
What are alkanes mainly used as?
fuels, exploiting their reaction with oxygen to generate heat
What is the general formula of alkanes?
CnH2n+2.
(double carbon number and add two to get hydrogen number)
What are the properties of alkanes we look into?
-Bonding
-Shape
-Variations in boiling points
-Effect of chain length on boiling points
-Effect of branching on boiling point
What is the bonding in alkanes?
-saturated hydrocarbons, containing only carbon and hydrogen atoms joined together by single covalent bonds
-each carbon atom is joined to four other atoms by a single covalent bond called a sigma bond (σ)
-σ bond is the result of the overlap of two orbitals, one from each bonding atom. Each overlap contains 1 electron, so σ bond has 2 electrons that are shared between the bonding atoms
-σ bond is positioned on a line directly between bonding atoms
-each carbon atom has 4 σ bonds (C-C or C-H)
What is the shape of alkanes?
-each carbon atom is surrounded by 4 electron pairs in 4 σ bonds which all repel each other
-repulsion results in a 3D tetrahedral arrangement around each atom
-each bond angle is approx 109.5°
-the σ bonds act as an axes around which the atoms can rotate freely, so the shapes are not rigid nut distance between bonds remains relatively the same
-evenly spaced carbon and hydrogen atoms
What is crude oil, how is it separated?
-contains hundreds of different alkanes
-oil refiners separate the crude oil into different fractions by fractional distillation in a distillation tower
-each fraction contains a range of alkanes
-this is possible due to the range of boiling points of different alkanes
What are the variations of boiling points of alkanes?
-all alkanes have different boiling points
-the greater the intermolecular forces (londons forces) the higher the boiling points
-once the forces are broke the molecules move apart from each other and the alkanes become a gas
In alkanes how does the length of the chain effect the boiling point?
-as the chain length increases, the molecules have a large surface area so more surface contact is possible between molecules
-the longer the chain = the greater the londons forces so more energy is required to overcome the forces
In alkanes how does branching have an effect on the boiling point?
-branched isomers of alkanes have lower boiling points then straight chain isomers
-the more branching there is = the fewer the surface points of contact between molecules, giving fewer lands forces
-the branches get in the way and prevent branched molecules getting as close together as straight chain molecules, decreasing the intermolecular forces further
What are the reasons for the lack of reactivity of alkanes?
-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
Why are alkanes used as fuels?
-give out heat
-readily available
-easy to transport
-burn in plentiful supply of oxygen without releasing toxic fuels
What is the equation that is used to balanced the complete combustion of alkanes?
CxHy + (x + y/4) O2 → xCO2 + y/2 H2O
What happens in the complete combustion of alkanes?
-react with a plentiful supply of oxygen to produce CO2 and H2O
what is the equation for the complete combustion of methane?
CH4 + 2O2 → CO2 + 2H20
What is the equation for the complete combustion of ethane?
C2H6 + 3.5 O2 → 2CO2 + 3H2O
What is the equation for the complete combustion of hexane?
C6H14 + 9.5 O2 → 6CO2 + 7H20
What happens during incomplete combustion of alkanes?
-limited supply of oxygen so alkanes burn incompletely
-forms carbon monoxide or soot (carbon itself)
What is the complete combustion and also the incomplete combustion to form CO and C in heptane?
Complete combustion (CO2 formed) - C7H16 + 11O2 ➝ 7CO2 + 8H2O
Incomplete combustion (CO formed) - C7H16 + 7.5 O2 ➝ 7CO + 8H20
Incomplete combustion (C formed) - C7H16 + 4O2 ➝ 7C + 8H2O
Where is a limited supply of oxygen most common?
In closed spaces: car engines, faulty heating systems, inadequate ventilation in living areas
What is the reaction of alkanes with halogens?
-only react with halogens in the presence of ultraviolet radiation
-UV light provides the initial energy for a reaction to take place
What is the reaction of methane with bromine?
-substitution reaction
-hydrogen atom is substituted by a halogen atom
CH4 + BR2 → CH3Br + HBr
What is the mechanism for the bromination of methane?
Radical Substitution
What are the three stages of the bromination of alkanes?
Initiation
Propagation
Termination
What happens during initiation (stage 1)? Use bromine as an example
-covalent bond is broken down by homolytic fission
-each atom (bromine) takes one electron from the pair forming two highly reactive (bromine) radicals
-energy for fission is provided by UV radiation
Br-Br → Br• + •Br
What happens during propagation (stage 2)? Use bromine as an example
-goes through two steps, a chain reaction
-radical is used and regenerated
-radical reacts with methane forming a methane radical and a molecule of HBr
-methyl radical reacts with another Br2 forming CH3Br and a new bromine radical
-in theory propagation steps could continue until all the reactants have been used up
Step 1: CH4 + •Br → •CH3 + HBr
Step 2: •CH3 + Br2 → CH3Br + Br•
What happens during termination (stage 3)? Use bromine as an example
-two radicals collide, forming a molecule with all electrons paired
-there are a number of possibilities of termination steps with different radicals in the reaction mixture
-when two radicals collide and react both radicals are removed from the reaction mixture, stopping the reaction
•Br + Br• →Br2
•CH3 + •CH3 → C2H6
•CH3 + •Br → CH3Br
What are the limitations of radical substitution in organic synthesis?
Further substitution
Substitution at different positions on a carbon chain
Why is initiation an example of homolytic fission?
one electron from the bond pair goes to each atom
