Question

Since unsaturated hydrocarbons are reactive they decolourize reddish brown bromine water by forming additions compound by addition reaction and not by substitution reaction.

Short answer

Question: Explain how unsaturated hydrocarbons cause the decolorization of reddish-brown bromine water and the type of reaction that occurs in this process. Answer: Unsaturated hydrocarbons, such as alkenes and alkynes, cause the decolorization of reddish-brown bromine water by undergoing an addition reaction. In this reaction, the double or triple bond between carbon atoms is broken, and bromine atoms are added to the carbon atoms. As the molecular bromine (Br2) reacts with the unsaturated hydrocarbon, its concentration in the solution decreases, causing the decolorization of the reddish-brown bromine water.

Step-by-step solution

Describe unsaturated hydrocarbons

Unsaturated hydrocarbons are organic compounds that contain at least one double or triple bond between carbon atoms, making them more reactive than saturated hydrocarbons that contain only single bonds. There are two types of unsaturated hydrocarbons: alkenes (containing a double bond) and alkynes (containing a triple bond).

Explain the reaction between unsaturated hydrocarbons and bromine water

When an unsaturated hydrocarbon reacts with bromine water (Br2 (aq)), the double or triple bond between the carbon atoms is broken, and bromine atoms add to the carbon atoms. This process is called an addition reaction. Bromine water has a reddish-brown color due to the presence of molecular bromine (Br2). As bromine atoms are added to the unsaturated hydrocarbon, the concentration of Br2 in the solution decreases, which results in the decolorization of the reddish-brown bromine water.

Show the reaction with a general representation of alkenes

Let's consider a general alkene represented by the formula R-CH=CH-R'. The addition reaction with bromine water can be represented as: R-CH=CH-R' + Br2 (aq) -> R-CH(Br)-CH(Br)-R' The double bond between the carbon atoms of the alkene has been broken, and two bromine atoms have been added to the carbon atoms, forming a new compound in which no double bonds are present.

Explain why no substitution reaction takes place

A substitution reaction would involve replacing an atom or group of atoms from the unsaturated hydrocarbon with bromine atoms, without breaking the double or triple bond. However, the reactive nature of unsaturated hydrocarbons and the high electronegativity of bromine favor the addition reaction, in which the double or triple bond is broken, and bromine atoms are added to the carbon atoms. This is why unsaturated hydrocarbons react with bromine water via an addition reaction, rather than a substitution reaction. In conclusion, unsaturated hydrocarbons react with bromine water through an addition reaction that breaks the double or triple bond between carbon atoms, forming an addition compound with incorporated bromine atoms. This reaction results in the decolorization of reddish-brown bromine water as the molecular bromine is consumed and its concentration in the solution decreases.

Key concepts

Addition Reaction

An addition reaction occurs when atoms or groups of atoms bond to a molecule without replacing any part of it. This is typical in the chemistry of unsaturated hydrocarbons—organic molecules that contain carbon-carbon double or triple bonds. These bonds are characteristically reactive due to their high electron density.

In the case of alkenes and alkynes, the double or triple bonds are able to 'open up', allowing new atoms to attach to the carbons. The classic example is the addition of halogens, like bromine, to alkenes or alkynes, where the molecule changes from being unsaturated to saturated as it gains additional atoms.

During an addition reaction, the product formed has the same number of carbon atoms as the original unsaturated hydrocarbon, but will have more hydrogen or halogen atoms depending on the reagents involved. It's essential to remember this reaction does not involve swapping one atom or group for another, distinguishing it from a substitution reaction.

Bromine Water Test

The bromine water test is a simple, yet powerful, qualitative analysis technique used to detect the presence of unsaturation in organic molecules. Bromine water—which has a distinct reddish-brown color due to the dissolved bromine—loses this color when it reacts with compounds containing double or triple carbon-carbon bonds.

In a laboratory setting, when a sample of alkene or alkyne is mixed with bromine water, the solution typically becomes colorless, signifying that an addition reaction has taken place: the unsaturated hydrocarbon has added the bromine atoms across its multiple bonds. This definitive change serves as a litmus test for unsaturation, and it's a favorite educational tool for illustrating reactivity in organic chemistry.

In a sense, the bromine water test serves not only a diagnostic purpose but also visually exemplifies concepts of bonding and reaction mechanisms, making it a handy reference point for students tackling the complexities of organic synthesis.

Alkenes and Alkynes

Alkenes and alkynes form the group of unsaturated hydrocarbons essential to organic chemistry. Alkenes are hydrocarbons with at least one carbon-carbon double bond (C=C), typified by the formula C_nH_2n. These are the simplest form of unsaturated hydrocarbons.

Alkynes, on the other hand, are characterized by at least one carbon-carbon triple bond (C≡C), and their general formula is C_nH_2n-2. The extra degree of unsaturation (compared to alkenes) makes alkynes even more reactive in certain chemical reactions, particularly addition reactions, due to the greater strain on the triple bond.

Both alkenes and alkynes exhibit isomerism, which includes both cis-trans (geometric) isomers and structural isomers, greatly expanding the diversity and utility of these compounds in chemical reactions. Their reactivity, particularly towards addition reactions, makes them crucial in synthetic organic chemistry, whether in creating polymers, pharmaceuticals, or other complex organic molecules.

Organic Chemistry

Organic chemistry is the scientific study of the structure, properties, composition, reactions, and synthesis of organic compounds primarily comprised of carbon atoms. This field of chemistry is vast and includes numerous classes of compounds, with hydrocarbons being the foundation.

In organic chemistry, understanding the nature of chemical bonds is vital, as it influences how molecules interact with each other. Reactions like addition and substitution are core to the discipline, shaping the synthesis of new compounds. Techniques such as the bromine water test are essential tools for identifying functional groups and understanding molecular reactivity, serving both analytical and pedagogical roles.

Moreover, organic chemistry is not just an academic pursuit; it has real-world applications in medicine, agriculture, energy, and materials science. It allows us to synthesize essential compounds, design drugs, create sustainable fuels, and so much more, making it a cornerstone of scientific and technological advancement.