4.2 Naming of Alkenes

Alkenes vs Alkanes

Alkanes are organic molecules we previously discussed that contain only single bonds.

Alkenes are “unsaturated” organic molecules containing at least one double bond on their parent chain.

• "Saturated" means all single bonds; no more hydrogens can be added.

What are the 4 base steps for naming alkenes

Step 1: Keep the alkane prefix but change the ending from "ane" to "ene."

Step 2: When numbering the atoms in the parent chain, give the location of the double bond the smallest possible carbon number, regardless of other substituents.

• include the location in between the name of the parent chain → But-3-ene

Step 3: Name the substituents alphabetically as usual. then the parent chain

Step 4: If there is more than one double bond, use "diene," "triene," etc. Include the location of each also in between the name, separated by commas (But-2,3-diene)

What is geometric isomerism

Geometric isomerism is a special case of isomerism only occurring for alkenes where the structural formula can exist as 2 distinct forms. Same structural formula but are spatially arranged differently.

• Only for alkanes because the rigid double bond cannot rotate, but the single bonds of alkanes can.

Geometric Isomerism: Trans vs Cis geometric isomers

Let's say we have two alkanes that have the same structural formula but are spatially arranged differently, how do we differentiate between them?

Draw an imaginary dotted line passing horizontally through the middle of the double bond

• If the other carbons are all above this line: CIS

• if the other carbons are NOT all above this line: TRANS

What are the three steps to drawing geometric isomers

step 1: start with the double bond

step 2: then cis/trans the rest of the parent chain

step 3: add any substituents

Between alkanes and alkenes, which are more reactive and why

Alkenes are very reactive compared to alkanes due to the stability and structure of the double bond(s)

Are alkanes and alkenes soluble in water

Neither are soluble in water

between alkanes and alkenes, which have a higher melting point and why

Alkanes have a higher melting/boiling point than alkanes due to greater IMFs between particles.

• This difference is very slight, however.

Melting point and boiling point increase in relation to what for both

• melting point increases in relation to increasing surface area

• boiling point increases in relation to increasing molar mass