Chapter 16- hydrocarbons Flashcards
What are the only two elements that make up hydrocarbons?
Hydrogen and carbon
Name and describe the four classifications of hydrocarbons.
Alkanes- fully saturated straight chain hydrocarbons (no double/triple bonds)
Alkenes- straight chain hydrocarbons with at least one double bond (unsaturated)
Alkynes- straight chain hydrocarbons with at least one triple bond (unsaturated)
Arenes- ringed hydrocarbons (unsaturated)
What are constitutional isomers?
Isomers that don’t have the same sequence of atom connections, e.g. a functional group is bonded to a different atom in the chain
What are conformational isomers?
Structures that can be made identical by rotation around one or more single bonds
Why are alkenes able to show stereoisomerism?
There’s no free rotation around the double bond, so where each molecule matters
What is steroisomerism?
In stereoisomerism, the atoms making up the isomers are joined up in the same order, but still manage to have a different spatial arrangement.
Define the term ‘cis isomer’
A stereoisomer where the functional groups are on the same side of the double bond
Define the term ‘trans isomer’
A stereoisomer where the functional groups are on opposing sides of the double bond
How is E-Z isomerism different to cis-trans isomerism?
E-Z isomerism is an extension of cis/trans notation that can be used to describe double bonds having two, three or four substituents.
When is a bond given the configuration E?
If the two groups of higher priority are on opposite sides of the double bond
When is a bond given the configuration Z?
If the two groups of higher priority are on the same side of the double bond
How is priority determined?
1) Compare the atomic number of the atoms directly attached to the stereocentre; the group having the atom of higher atomic number receives higher priority.
2) If there is a tie, we must consider the atoms at distance 2 from the stereocentre- the side with the highest mass number has the higher priority etc
Physical properties of alkanes
State is dependent on length of chain. Shorter chains are gases at RTP, longer chains are solids. Branched alkanes normally exhibit lower boiling points than unbranched alkanes of the same carbon content. They are almost completely insoluble in polar solvents
Physical properties of alkenes
The physical properties of alkenes are very similar to those of alkanes. Alkenes also exist as gases, liquids, and solids at room temperature. Isomeric alkenes tend to have similar boiling points, which makes it difficult to separate them by boiling point differences.
Physical properties of alkynes
The physical properties of alkynes are very similar to those of the alkenes. Alkynes are generally nonpolar molecules with little solubility in polar solvents, such as water. Solubility in nonpolar solvents, such as ether and acetone, is extensive. Like the alkanes and alkenes, alkynes of four or fewer carbon atoms tend to be gases.
