Chemical Reactivity of Hydrocarbons Course Notes

Hydrocarbon Structure and Classification

Hydrocarbons contain only carbon and hydrogen. They can be classified as either aliphatic or aromatic hydrocarbons.

Hydrocarbon classification

Image from St. Paul's Catholic High School at

Kinetic Stable

When a chemical reaction occurs either extremely slowly or not at all because of the absence of a reaction mechanism the reactants are said to kinetically stable. This stability is not associated with low energy but rather it is a stability that a high energy molecule has because no reaction mechanism is available by which a low energy molecule can be produced.

Thermodynamic Stability

This type of stability results when a substance is a low energy substance.

Hydrocarbon Solubility

The rule to use when determining hydrocarbon solubility is: Like dissolves like.

This means that polar compounds (water, alcohols, and carboxylic acids) dissolve other polar compounds. Nonpolar compounds dissolve other nonpolar compounds but tend not to dissolve polar compounds.

When you test for solubility you are looking for either a homogeneous solution or a heterogenous solution. Homogeneous solutions have no layers evident indicating the hydrocarbon being tested is soluble. These hydrocarbons are miscible. Heterogenous solutions have layers evident indicating the hydrocarbons are insoluble. These hydrocarbons are immiscible.

Unsaturation and Alkanes and Alkenes

Alkanes are considered saturated because they have only single C-C bonds and cannot add a hydrogen. Alkenes are unsaturated because they are capable of adding a hydrogen when the C=C double bond is broken.

Bromine Test for Hydrocarbon Reactivity

The bromine test is used to determine if the colorless organic compound contains any double C=C bonds (the alkene functional group). Bromine does not react with an alkane because the alkane contains only single C-C bonds which cannot add the bromine. Alkanes merely dilute the red-brown bromine color to an orange or yellow color in the absence of a strong catalyst.

Due to their C=C double bonds which can be broken, alkenes react readily with bromine to produce saturated dibromoalkanes. When an alkene is reacted with bromine, the red-brown color of the bromine is immediately lost due to the reaction of the bromine.

Alkane + bromine (No strong light or heat) results in diluted solution colored orange or yellow indicating no reaction.

Alkane + bromine (heat or strong light acting as a catalyst) results in the brown red color of bromine slowly disappearing.

Alkene + bromine results in the red brown color of bromine rapidly or immediately disappearing giving a colorless solution

Aromatic hydrocarbon + bromine (no heat or light acting as a catalyst) results in no reaction and the red brown color of bromine is diluted to orange or yellow.

Aromatic hydrocarbons are too stable to react without a catalyst so they act like alkanes.


Red brown color of bromine

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 bromine test for unsaturation

Bromine Test (left) No reaction, saturated, (right) Reaction, unsaturated

Bromine test image from

If an alkene is present, the π bond breaks and bromine is added in two places.

alkene bromine reaction



Cholesterol has only one -OH group which is polar. This single alcohol functional group does not make it polar enough to dissolve in water or blood (made up of mostly water). Since cholesterol is not polar enough to dissolve in water it will dissolve in nonpolar solvents. Cholesterol when tested with the bromine test will react, losing the red-brown color of bromine and turning colorless. This is due to the one double (π) bond. This double bond can break allowing bromine to be added, indicating an alkene is present and cholesterol is unsaturated allowing the red brown color of bromine to slowly disappear.

Red brown color of bromine slowly disappears = unsaturated.

Red brown color of bromine dilutes to yellow or orange = saturated.

Bromine test for Unsaturation

Again bromine is used to determine whether an alkane, alkene, or aromatic hydrocarbon is present. If the substance is an alkene it will react with the bromine, indicating the compound is unsaturated. When the red-brown color of bromine disappears it indicates the substance is unsaturated and a reaction has occurred.

ALKANES contain only σ (single C-C) bonds and cannot add any Br so red brown color of bromine dilutes to yellow or orange = saturated.

ALKENES contains a π bond making it possible to add Br so the red brown color of bromine disappears rapidly or immediately = unsaturated.

AROMATIC HYDROCARBONS are extremely stable and will not react without a catalyst = saturated.

When the red-brown color of bromine is diluted to yellow or orange (no reaction), the substance is saturated.

Red-brown color of bromine is diluted to yellow or orange (no reaction), the substance is SATURATED.

Red brown color of bromine disappears (reaction), the substance is UNSATURATED.

Digital Video from DVAction of the Bromine Test for Alkenes

Baeyer Test for Unsaturation

The Baeyer reagent is a cold dilute aqueous solution of potassium permanganate which is a deep purple color. Potassium permanganate does not react with alkanes because they are saturated (single bonds which are all taken). When it is added to alkanes the purple color does not change. However, when it is added to an alkene, the purple color slowly disappears and a brown MnO2 precipitate forms. The appearance of the brown precipitate indicates a positive test for unsaturation. The Baeyer test for unsaturation is used when the color of the organic compound may interfere with the result of the Bromine Test for Unsaturation.

Purple color dilutes to light purple = SATURATED.

Purple color disappears and brown ppt forms = UNSATURATED.


The purple color of KMnO4

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baeyer test

Baeyer Test for Unsaturation (left) purple diluted to light purple: saturated, (right) purple disappears, brown preciptate forms: unsaturated.

Image from

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last updated: Jan 16, 2007