CH0003 - Lecture 1 - Hydrocarbons
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What are hydrocarbons?
How can they be classified?
A hydrocarbon is an organic compound consisting exclusively of hydrogen and carbon.
They can be saturated (alkanes) and unsaturated (alkenes, plus others)
What are alkanes?
Described as "saturated hydrocarbons - non-ring, and have the general formula of CnH2n+2
All of the carbons are bonded to at least one other carbon (except in methane) by single bonds, and the carbons carry the maximum number of hydrogens.
They contain only carbon-hydrogen bonds and carbon-carbon single bonds.
All carbon bonds are sp3
What are cycloalkanes?
Cycloalkanes again only contain carbon-hydrogen bonds and carbon-carbon single bonds, but this time the carbon atoms are joined up in a ring.
If you count the carbons and hydrogens, you will see that they no longer fit the general formula CnH2n+2. By joining the carbon atoms in a ring, you have had to lose two hydrogen atoms. CnH2n
What are alkenes?
Described as "unsaturated" hydrocarbons with the general formula of CnH2n (for mono alkene)
Formed by the removal of two hydrogens from adjacent carbons. Contain a carbon-carbon double bond - sp2
What are the carbon codes for 1-12 carbons?
- 1 = Meth
- 2 = Eth
- 3 = Prop
- 4 = But
- 5 = Pent
- 6 = Hex
- 7 = Hep
- 8 = Oct
- 9 = Non
- 10 = Dec
- 11 = Undec
- 12 = Dodec
What are the two codes for the types of carbon bonds, and give two examples.
- an = only carbon-carbon single bonds.
- en = contains a carbon-carbon double bond.
E.g. butate means four carbons in a chain with no double bond
E.g. propene means three carbons in a chain with a double bond between two of the carbons.
What are alkyl groups? Give examples
- Compounds like methane, CH4, and and ethane, CH3CH3 are alkanes.
- If you remove a hydrogen atom from one of these you get an alkyl group.
- For example:
- A Methyl group is CH3
- An Ethyl group is CH3CH2
These groups must, of course, always be attached to something else.
How would you draw 2-methylpentane?
Pent means 5 carbons - draw a five carbon skeleton.
an means no carbon carbon double bonds
2-methyl means there is a methyl group on the 2nd carbon - add that.
Add the remaining hydrogens so each carbon has 4 bonds.
How would you draw but-1-ene?
But means 4 carbons
en means there is a double bond
1 tells you the double bond starts at 1 carbon
All alkanes end with the suffix...
Comment on alkane isomerism.
All the alkanes with 4 or more carbon atoms in them show structural isomerism.
This means that there are two or more different structural formulae that you can draw for each molecular formula.
e.g. C4H10 could be butane or it could be 2methylpropane
What is full structural drawing of molecules?
- When you show all the bonds in the molecule as individual lines.
- You need to remember that each line represents a pair of shared electrons.
What is skeletal formula drawing?
In skeletal formula, all the hydrogen atoms are removed from carbon chains, leaving just a carbon skeleton with functional groups attached to it.
- In a skeletal diagram:
- - There is a carbon atom at each junction between bonds in a chain and at the end of each bond (unless there is something else there already - like an -OH group)
- - There are enough hydrogen atoms attached to each carbon to make the total number of bonds on that carbon up to 4.
Comment on the boiling/melting points of alkanes.
Alkanes are non-polar molecules i.e. they have no net dipole or charges. Therefore they experience only inter-molecular Van der Waals forces (VdW), which are weak.
The magnitude of VdW forces is proportional to the number of electrons within a molecule, and therefore as molecular mass increases, so do VdW bonding forces. The consequence of this is seen in the trends in the boiling/melting points of the alkanes.
The stronger, or the greater amount of VdW forces within any given alkane will result in a larger boiling point. This effect can explain the trend of boiling points within a homologous series of alkanes. (i.e. a series where all other properties are equal, e.g. all linear).
What is a general rule of thumb for the boiling points of alkanes?
The boiling point rises 20-30°C for each additional carbon added.
How do boiling points differ between linear and branched alkanes?
A straight chain molecule alkane has a higher boiling point than a branched chain alkane with the identical number of carbons.
Boiling point of alkanes depends on VdW forces. Linear molecules can easily stack together and closely
Branching results in the molecules loosing the ability to fit closely together i.e Poor stacking
VdW is proportional to the distance between the molecules, inter-molecular distance.
Therefore branching decreases the VdW forces and results in a lower boiling point.
Comment on the solubility of Alkanes.
Non-polar (electronegativity C = 2.5 and H = 2.2, very similar no dipole)
Soluble in other non-polar solvents.
Insoluble in water (water is very polar due to hydrogen bonding and inherent dipole – “Like dissolves like”)
Comment on the reactivity of alkanes.
Alkanes are essentially inert under most conditions. The C-C and C-H single bonds are very non-reactive.
Free radical halogenation is one of the few reactions possible (see later on in course)
Catalytic cracking (crude oil processing) where the large hydrocarbon molecules are cracked into smaller more useful molecules (e.g. petrol). High temperature, high pressure and special catalysts needed (atypical reaction conditions)
What are alkenes?
Alkenes are molecules that contain the C=C bond, i.e. a carbon-carbon double bond. This is termed a functional group, it is one of the most simple but most widespread.
Comment on the reactivity of alkenes
The C=C bond is reactive, and as a result alkenes (unlike alkanes) can undergo a huge variety of reactions.
How does degree of unsaturation relate to Double Bond Equivalents (DBE)
Some molecules can contain more than one double bond – for any given molecular formula this has to be calculated to draw a structure. E.g. C6H8
This is easy to work out by drawing the possibilities, but for larger molecules it is not so easy!
Instead of calling it degree of unsaturation (which is only applicable to linear hydrocarbons) we use Double Bond Equivalents (DBE)
How can you calculate DBE?
This can be derived by calculating the maximum number of Hydrogens (Hmax) using the formula below. Then the double bond equivalents can be calculated using the second formula
Hmax= 2C + 2 + N;
DBE = (Hmax- H)/2
- H = No. hydrogens
- C = No. carbons
- N = No. nitrogens
Why express the number of double bonds as DBEs? If you calculated the DBE for any given formula, it doesn’t mean that all DBEs are double bonds! If you look at the cycloalkanes, their general formulae follow CnH2n(n>3), i.e. it seems like they all have one double bond!
The formation of a ring results in the ‘loss’ of two hydrogens, just like a C=C bond does. So a single ring system has (at least) one DBE
Why are DBE so useful?
If we had an unknown substance and some experimental data on this substance (molecular mass, empirical formula) we can use DBE to help in the assignment of structure. It gives information on other functional groups too.
It can also be used to check whether a chemical formula is theoretically possible! E.g. C20H53
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