Alkanes
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What does displayed formula show?
Shows the relative positioning of atoms and the bonding between them
What does structural formula show?
It shows the arrangement of atoms but not the bonds between them ie butane CH3CH2CH2CH3 or you can do CH3(CH2)2CH3
What do brackets show in a structural formula?
They show the atoms are on a side chain or there is multiple of them.
What does skeletal formula show?
Simplified organic formula removing hydrogen atoms leaving just the carbon atoms and their associated functional groups
If a compound has a benzene ring what does this mean?
It means it is an aromatic
If a compound has a ring other than a benzene ring what is the compound?
It is alicyclic but also aliphatic
Alicyclic
Has a ring other than benzene ring
Aliphatic
Has no benzene ring
Whats different from a benzene ring and other rings?
Benzene- has double/triple bonds Others- don’t
What’s the order of the first 10 alkanes? Name of first 10 side chains?
Methane- methyl Ethane- ethyl Propane- propyl Butane- butyl Pentane- pentyl Hexane- hexyl Heptane- heptyl Octane- octyl Nonane- nonyl Decane- decyl
How to name alkanes?
• Find the longest carbon chain • Identify side chains, if there is two carbons in a side chain then its a methyl if 4 then a butyl • Number the carbon atoms on the main chain, starting from the carbon atom closest to a side chain • Write out the name, with the side chains as prefixes, put a number before the prefix to show the carbon atom it is bonded to. If you have two of the same group, write out the number and di before the prefix ie 2,3-dimethyl
Rules for naming alkanes.
• Prefixes go in alphabetical order • Put di/tri before to show if there is 2/3 of the same side chain ie 2,3,4-trimethyl • Dash between numbers and letters and commas between two numbers
What is a homologous series?
A series of organic compounds with the same functional group but with each successive member differing by CH2
What is a functional group?
A group of atoms responsible for the characteristic reactions of a compound.
How is the rotation of a sigma bond?
Free rotation
What is an alkane?
A compound containing only carbon and hydrogen atoms with only C-C single bonds and C-H bonds.
Why and what is the shape around each carbon atom in an alkane?
A tetrahedral as it has four bonding pairs/bonding regions. Bonding pairs of electrons repel each other as much as possible causing the shape to be a tetrahedral and the angle to be 109.5 degrees.
Why is the reactivity of alkanes low?
C and H are similar in electronegativity so it is not polar however it can undergo combustion reactions.
How does boiling point vary with carbon hydrogen chain length?
The longer the chain, the higher the boiling point. This is because there are more electrons/there is higher surface area of contact which makes the london forces stronger thus require more energy to overcome.
How does branching affect surface area of contact and therefore boiling point?
The more branched a molecule is, the lower the surface area of contact and so weaker london forces which need less energy to overcome resulting in a lower boiling point.
State the general formula of an alkane
CnH2n+2
State the two types of combustion reactions that alkanes take place in + conditions.
Alkane + oxygen———> carbon dioxide + water (COMPLETE COMBUSTION WHEN THERE IS ENOUGH OXYGEN PRESENT) Alkane + oxygen——-> carbon monoxide + water (INCOMPLETE COMBUSTION WHEN THERE ISN’T ENOUGH OXYGEN PRESENT)
Why is incomplete combustion dangerous?
Produces carbon monoxide, a toxic gas.
Rules for balancing combustion equations.
• Balance carbon first • Balance hydrogen • Then balance oxygen- typically put a fraction before it