I remember noticing that bananas in this country seem to ripen faster than in India. Two days ago the banana was greenish-yellow and today it is completely yellow with brown spots, why?
Most of the bananas in the United States are picked unripe and ethylene gas is passed over them to hasten the ripening process.No wonder half green and yellow bananas turn ripe so quickly. In fact, during the natural ripening of fruits, ethylene gas is given off. What is ethylene, also known as ethene? It is the first in the alkenes (also called olefins/olefines) series I will talk about today.
(Week 3 was about alkanes.) Alkenes have the general formula CnH2n and the first one in the series is C2H4 , called ethylene or ethene, (second picture above)
Notice the two bonds connecting the two carbon atoms. Let us take a few minutes to understand what the word ‘bond’ means. There are different kinds of chemical bonds; we have seen hydrogen bonding and today we will look at covalent bonding. Organic compounds exhibit covalent bonding.
That line or toothpick (Week 3) I use to indicate a bond is actually the sharing of electrons here. All chemical reactions involve transfer of electrons and all compounds in organic chemistry consists of the sharing of electrons. Here with only C-H bonding you have the sharing of two electrons.The carbon atom has one electron that it shares with the one electron that the hydrogen atom has leading to two shared electrons;but since there is a double bond for the C-C bond, there are actually 4 electrons here. ( If it had been a single bond as in ethane, there would be the sharing of two electrons for the C-C bond.)These two shared electrons comprises the covalent bond indicated by lines (or dots for the electrons) like shown above in ethylene or ethene.
Since we are dealing with double bonds between two carbon atoms, the first in the alkene series has to have two carbon atoms.When n=3 we have C3H6, called propene and when n=4 it is called butene, C4H8. When there are 4 carbon atoms, for butene, you can have the double bond between the carbon 1 and 2 and create two distinct structures. First, you can have a straight chain structure and then a branched chain structure for the carbon atoms. (You need a minimum of 4 carbon atoms to do straight and branched chains of carbon atoms.
Next, the double bond can move between the second and third carbon atoms. Now, since we have a double bond, the structure cannot rotate so positions for the attachments to the second and third carbon atoms become rigid. When the methyl ( methane is CH4 ,but when one hydrogen is less it becomes methyl written as CH3) components are on the same side it is called ‘cis’ and when they are on opposite sides it is called ‘trans’. Observe propene ( only one form) and butene with 4 isomers:
Notice that the carbon atom has 4 bonds while the hydrogen atom forms only one bond. If you have a 5 carbon alkene,it will be pentene, and you will have more isomers possible. Many of these isomers are extremely important; slight variation in structure causes immense changes in properties. Week 11 we will look at some remarkable examples of isomers and their significance in medicine, especially.
Week 3 I covered alkanes, where the carbon atom had no double bonds, today we see a double bond in each of the alkenes mentioned. Hydrocarbons like alkanes are called saturated hydrocarbons and when a double bond or bonds are present in an organic molecule, it is called unsaturated. Oils are usually unsaturated which means they are organic molecules with double bonds while solids like butter, lard are saturated with no double bonds. In daily life we hear the words ‘saturated fats’, ‘unsaturated fats’ and now you can at least know that the chemical structure includes single or double bonds between carbon atoms.
Alkanes are vital as fuels while alkenes are the building blocks for the manufacture of plastics. Ethene, we already know is used in the artificial ripening of fruits.
Activities for Middle School Teachers: Let students construct alkenes from n=5 to say, n=10. Let students do straight chain and branched chain for the carbon atoms and also look for the different isomers.
Let students look for a relationship between number of carbon atoms and number of isomers, is that a linear relationship? ( What is a linear relationship?)
Find out if other countries ripen fruit artificially, what chemicals do they use besides ethylene/ethene? Locate these countries in a map.
Students can study how effective unsaturated fats are compared to saturated fats for a healthy diet.
Nuggets of information: A diene is an organic compound with two double bonds. Butadiene is a four-carbon hydrogen compound with two double bonds. The formula is C4H6 and the carbon double bonds are in the 1, 3 position: -C=C-C=C-
Butadiene is extremely useful today; it is the starting point for the manufacturing of synthetic rubber (Week 6)
Trans fats today are even worse for our diets than saturated fats. The adding of hydrogen to unsaturated oils leads to a solid product like margarine where the hydrogen atoms are on opposite sides of a double bond in the oil structure. ( trans position).
References:
Chobhi's Blog 