What are the differences between alkenes and alkynes? What are the differences between cis- and trans-isomers? How are cistrans isomers used for night vision? What is a haloalkane? Identify one of them. How does it function as an anesthetic? Describe in detail.
Both alkenes and alkynes are unsaturated hydrocarbons. Alkenes possess C-C double bond, formed through sp2 hybridization, and are planar in structure, but alkynes possess C-C triple bonds, formed through sp hybridization, and are cylindrical in shape. Alkynes have acidic hydrogens, and hence they react with Cu2Cl2 in ammonia to produce a red precipitate, while alkenic hydrogens do not exhibit such acidity. On the other hand, alkenes exhibit cis-trans isomerism, which alkynes do not.
In alkenes, the cis isomers have similar groups on the same side of the double bond, while trans isomers have similar groups on the opposite side of the double bond. They differ in physical and chemical properties. Cis isomers have higher dipole moment and boiling point while trans isomers have comparably higher m.pt. and density.
Cis-trans isomerism plays an important role in the reception of light by the retina of eye. Within our retina, microscopic structures called rod cells and cone cells contain a compound called retinal, which absorb light. The arrangement of C-C double bonds gives retinal a shape that fits the protein opsin, to which it is attached (refer to the attached image).
In the darkness, rhodopsin (opsin + retinal) is inactive (or stable), cGMP is high and ion channels are open. When light strikes retinal, 11-cis retinal bound protein is converted to 11-trans retinal bound protein, which no longer fits the protein opsin and is released. Accompanying this release an electrical impulse is sent to the brain which facilitates the vision process. In the recovery phase, i.e. in the dark or during blinking of the eyelids, 11 cis-retinal recombines with opsin to regenerate the active photopigment.