Unsaturation and Reactivity: The presence of the C=C double bond makes alkenes 'unsaturated' because the carbon atoms are not bonded to the maximum possible number of other atoms. This allows them to form additional bonds without losing any existing atoms, unlike alkanes which are 'saturated' and can only undergo substitution reactions.
Electron Density: The C=C double bond consists of a strong -bond and a weaker -bond. The -bond has a high concentration of electrons above and below the plane of the carbon atoms, creating a region of high electron density. This electron-rich region is susceptible to attack by electrophiles (electron-deficient species), such as the slightly positive end of a polarized bromine molecule.
Mechanism Overview: In the presence of an alkene, the nonpolar bromine molecule () becomes polarized as it approaches the electron-rich double bond. One bromine atom acts as an electrophile, forming a bond with one carbon of the double bond, while the other bromine atom leaves as a bromide ion. Subsequently, the bromide ion attacks the carbocation intermediate, leading to the formation of the dibromoalkane.
Purpose: The bromine water test is a simple and effective chemical test used to distinguish between saturated alkanes and unsaturated alkenes. It relies on the difference in their reactivity towards halogens.
Reagent: The test uses bromine water, which is an aqueous solution of bromine. It is preferred over pure bromine for safety and ease of handling, and it has a characteristic orange-brown color.
Procedure: A small amount of the unknown hydrocarbon is added to bromine water and shaken. The observation of a color change (or lack thereof) indicates the presence or absence of a C=C double bond.
Observation with Alkenes: When bromine water is added to an alkene, the orange-brown color of the bromine water rapidly disappears, or 'decolorizes'. This is because the bromine molecules react with the alkene via an addition reaction, consuming the free bromine in the solution.
Observation with Alkanes: When bromine water is added to an alkane, no reaction occurs under normal conditions (e.g., in the absence of UV light). Consequently, the orange-brown color of the bromine water persists, indicating the absence of a C=C double bond.
Structural Difference: Alkenes possess at least one carbon-carbon double bond (C=C), making them unsaturated, while alkanes contain only carbon-carbon single bonds (C-C), making them saturated. This fundamental structural difference dictates their chemical behavior.
Reactivity: Alkenes are significantly more reactive than alkanes, particularly towards addition reactions. The electron-rich C=C double bond acts as a nucleophile, readily reacting with electrophiles like bromine.
Reaction with Bromine: Alkenes undergo rapid addition reactions with bromine (or bromine water), leading to decolorization. Alkanes, on the other hand, do not react with bromine water under ambient conditions; they only react with bromine in the presence of UV light via a slower substitution reaction, which does not cause immediate decolorization.
Identify the Functional Group: Always look for the C=C double bond when identifying alkenes. Its presence is the key to understanding their reactivity and predicting reaction outcomes.
Predict Products: For bromination, remember that two bromine atoms add across the double bond. If you start with ethene (), the product will be 1,2-dibromoethane (). Ensure the numbering correctly indicates the positions of the bromine atoms.
Bromine Water Test Interpretation: The key observation is the decolorization of orange/brown bromine water. If the color persists, it's likely an alkane. If it disappears, it's an alkene. Be precise in describing the color change.
Explain Reactivity: When asked why alkenes are more reactive, refer to the high electron density of the -bond in the C=C double bond, which makes it susceptible to electrophilic attack.
Confusing Addition and Substitution: A common mistake is to describe the reaction of bromine with an alkene as substitution. Remember, alkenes undergo addition, breaking the -bond and adding atoms, while alkanes undergo substitution, replacing an atom (usually hydrogen) with another.
Incorrect Product Naming: Students sometimes forget to include 'dibromo' or incorrectly number the positions of the bromine atoms in the product. Always ensure the product name reflects the two bromine atoms and their locations.
Misinterpreting Bromine Water Test: Some students might incorrectly state that alkanes react slowly with bromine water or that the color change is subtle. The reaction with alkenes is rapid and clear decolorization; alkanes show no reaction with bromine water under test conditions.
Mechanism Details: While the full electrophilic addition mechanism might be advanced for some curricula, understanding that the -bond breaks and new single bonds form is crucial. Avoid thinking of it as a simple bond swap.
Other Halogens: The addition reaction is not exclusive to bromine. Other halogens like chlorine () and iodine () also undergo similar addition reactions with alkenes, forming dichloroalkanes and diiodoalkanes, respectively.
Other Addition Reactions: Alkenes can undergo other types of addition reactions, such as hydrogenation (addition of hydrogen, ), hydration (addition of water, ), and hydrohalogenation (addition of hydrogen halides, ), each forming different saturated products.
Polymerization: The ability of alkenes to undergo addition reactions is also fundamental to their use in polymerization, where many alkene monomers add together to form long polymer chains, such as poly(ethene) from ethene.
