Alkenes

General Information:

Alkenes are a class of hydrocarbons (molecules containing only carbon and hydrogen atoms), being unsaturated compounds with at least one carbon-to-carbon double bond. Their typical chemical formula is CnH2n.

Physical Properties:

At room temperature, alkenes exist in all three phases: solids, liquids, and gasses. For example, Ethene, Propene, and Butene exist as colorless gasses. alkenes with 5 or more carbons such as Pentene, Hexene, and Heptene are liquid, and Alkenes with15 carbons or more are solids.

Alkenes are lighter than and insoluble in water due to their non-polar characteristics. As such, alkenes are only soluble in nonpolar solvents. For example, though being virtually insoluble in water, they can dissolve in organic solvents.

Alkenes display weak dipole-dipole interactions due to the electron-attracting sp2 carbon. The boiling point of each alkene is very similar to that of an alkane (hydrocarbons with only carbon-to-hydrogen bonds) with the same number of carbon atoms. The boiling points of alkenes depend on more molecular mass (chain length), in which the more intermolecular mass is added, the higher the boiling point is. Meanwhile, the intermolecular forces of alkenes get stronger with an increase in the size of the molecules.

Chemical Properties:

Combustion

Alkenes can combust, but they are less likely than alkanes to combust completely.

Complete combustion of alkenes produces carbon dioxide and water, provided there is a plentiful supply of oxygen.

CnH2n + (1.5 n)O2        nCO2 + nH2O

Incomplete combustion of alkenes occurs where oxygen is limited, producing water, carbon monoxide, and carbon (soot). This creates a smoky flame.

Addition Reactions:

Additional reactions occur when one alkene molecule combines with another alkene molecule, forming one large molecule and no other products. The C=C functional group allows alkenes to undergo these reactions.

e.g. 1 Halogenation:

Ethene reacts with bromine to form 1,2-dibromoethane

CH2=CH2 + Br2 → CH2BrCH2Br

e.g. 2 Hydrogenation:

Ethene reacts with hydrogen to form ethane (with Pd catalyst)

CH2=CH2 + H2 → C2H6

e.g. 3 Hydration:

Ethene reacts with steam to form ethane (with Pt catalyst & high temperature)

CH2=CH2 + H2O → CH3CH2OH

Reference:

“Alkenes.” Chemistry LibreTexts, 13 Sept. 2020, https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Alkenes.

“Testing for Alkanes and Alkenes - Organic Chemistry - (CCEA) - GCSE Chemistry (Single Science) Revision - CCEA - BBC Bitesize.” BBC News, BBC, https://www.bbc.co.uk/bitesize/guides/zmbyqp3/revision/8.