Alkanes Flashcards
Why does the boiling point of alkanes increase as the carbon chain length increases
As chain length increases , the surface area of the molecule increases so there is a greater surface contact possible between the molecules so there will be greater /stronger London forces and so more energy is needed to overcome them . Additionally number of electrons increases which increases London forces
Why do branched chain alkanes have a lower boiling point than straight chain alkanes
Branched alkanes have fewer surface points of contact between molecules meaning there will be fewer London forces . Also because of the shape of the branched molecule the molecules cannot get as close together or pack closely together whereas straight chain alkanes can . This further decreases the intermolecular forces . The result is less energy will be needed to overcome the forces
What are sigma bonds
Sigma bonds form when electron orbitals from adjacent atoms directly overlap and they contain a pair of electrons
What is the reactivity of the alkanes and why
Alkanes are generally un reactive because of the strong covalent bonds within alkane molecules . Additionally organic reactions occur with the attraction to part of an organic molecule which carries some positive or negative charge . Alkanes don’t have a separation of charge therefore they have a fairly restricted set of reactions
Why do alkanes not have a seperation of charge
Because carbon and hydrogen atoms have a very similar electronegativity so they are non polar molecules
Are the bonds polar in alkanes
The carbon-hydrogen bonds are only very slightly polar so there is no significant amount of positive or negative charge which other things might be attracted to
What are alkanes
Alkanes are saturated hydrocarbons , containing only carbon and hydrogen atoms joined together by a single covalent bond
What is the type of covalent bond alkanes have
A sigma bond , which is the result of the overlap of two orbitals one from each bonding atom and both overlapping orbital contains one electron so the o bond has two electrons that are shared between the bonding atoms .
Where is an o bond positioned
An o bond is positioned on a line directly between bonding atoms
How many sigma bonds are there
Each carbon atom in an alkane has 4 sigma bonds (either c-c or c-h)
What is the shape of alkanes and why
Each carbon atom is surrounded by 4 electron pairs in 4 o bonds , repulsion between these electron pairs results in a 3D tetrahedral arrangement around each carbon atom . Each bond angle is 109.5 and the o bonds act as axes around which the atoms can rotate freely , so these shapes are not rigid
How is crude oil separated and how
Separated into fractions by fractional distillation. This is possible because the boiling points of the alkanes are different , increasing as their carbon chain increases
What do the London forces do
The London forces hold the molecules together in solids or liquids but once broken the molecules move apart and the alkane becomes a gas
What is 2 features of sigma bonds
They are fully rotational and strong
Why are alkanes insoluble in water
Because they have no permanent dipole and can’t form hydrogen bonds
What reactions can the alkanes do
- combustion
-react with halogens
-thermal decomposition reactions (cracking )
What happens when hydrocarbons are burnt in a limited supply of oxygen
Incomplete combustion occurs and CO and H20 are produced
What happens if there is even less oxygen available
Then solid carbon may form . Which is a sooty flame and creates particulates so C and H20 form
Why is carbon monoxide a toxic gas
Because it binds irreversibly with haemoglobin
What is bad about carbon particulates
Exacerbate asthma and are carcinogens
What is halogenation of alkanes
When a halogen reacts with an alkane to produce a haloalkane
How are haloalkanes formed
Haloalkanes can be formed by exposing a mixture of an alkane and a halogen to uv light causes a reaction mechanism to occur resulting in a haloalkane forming
Why would we not use fluorine or iodine
Fluorine is too reactive and would be likely to explode and iodine is too unreactive
What is halogenation an example of
A substitution reaction
What are the 3 stages in a free radical reaction mechanism
Initiation , propagation and termination
What is the definition for initiation
The first step in a radical substitution in which the free radicals are generated by homolytic fission of a covalent bond , by uv light
What is the definition for propagation
Will always have a radical among the reactants and produce another radical in the products causing a chain reaction
What is the definition for termination
Is the step at the end of a radical substitution when two radicals combine to form a molecule . No more radicals are made
What is the definition for radical substitution
Is a type of substitution In which a radical replaces a different atom or group of atoms
What are the reagents for the chlorination of methane
Chlorine and methane
What are the conditions
Uv radiation
What is the equation for the chlorination of methane
CH4+Cl2—> CH3Cl + HCL
How does this occur
Methane is attacked by cl free radicals resulting in a hydrogen atom being substituted by a chlorine atom . The products are hydrogen chloride and chloromethane
What is this mechanism known as
Free radical substitution
What are the steps for the chlorination of methane
1) initiation
Cl2 —> 2Clº
2) propagation
Clº + CH4 —> ºCH3 + HCL
ºCH3 + Cl2 —-> CH3Cl + ºCl
3) termination
Clº + ºCH3–> CH3Cl
Clº + Clº —> Cl2
ºCH3 + ºCH3 —> C2H6
What are the limitations of free radical substitution
If an alkane is more than 2 carbons in length then substitution may occur on different carbon atoms , as any of the hydrogen atoms may be substituted, leading to a mixture of different isomers
Additionally the mixture of products is hard to separate and this is one reason why chain reactions are not a good method of preparing haloalkanes
Another one is further substitutions can occur until all hydrogens are subsituted , these further subsituted chloroalkanes are impurities that must be removed
How can we reduce the impurities
Decrease the risk by reducing the amount of chlorine or halogen in mixture or increase amount of organic molecule
How can you separate the different different products
Fractional distillation
