Science Makes Sense-Week31:Organic Chemistry, Cyclic compounds, alicyclic and aromatic.

When people visit young mothers with little babies in India, they first place a little sugar in the mouth of the baby and then give money. The baby is not interested in the money, but the little one lights up when the sugar hits the palate! Sugar, chemically, is a cyclic hydrocarbon compound.
Today we shall look at simple cyclic hydrocarbons, their general formula, nomenclature and some well-known cyclic hydrocarbons including sugars with different groups attached. We shall look at their uses and properties and briefly touch on aromatic compounds as well.
Cyclic hydrocarbons are hydrocarbons that form a ring. The simplest of cyclic hydrocarbons are cyclo-alkanes, which are saturated hydrocarbons. Let us start with the smallest ring (looking like a triangle) that will have 3 carbons and 6 hydrogen atoms called cyclo-propane with the formula C3H6. The next one in the series will be the 4-membered ring, cyclo-butane,C4H8, resembling a quadrilateral; the third one will be a 5-membered ring called cyclopentane, C5H10.(Ref.1) As one can see, the general formula resembles a straight-chain alkene and not a straight-chain alkane. Most cyclo-alkanes have similar reactivity to their corresponding alkanes.(Ref.2)
In cyclo-alkanes the carbon-carbon bond is sp3 hybridized and the ideal angle is 109.5 degrees, but in cyclo-propane and cyclo-butane the angles are 60 and 90 degrees respectively, making them unstable and reactive molecules.(Ref.1)
Sometimes these cyclic compounds might even have straight-chain hydrocarbons attached to them and then they are called generally by the term alicyclic compounds. The straight-chain hydrocarbons along with the saturated cyclic hydrocarbons are classified as aliphatic compounds(Ref.3)
Sugars are also cyclic compounds, but they are not cyclic hydrocarbons. Just like the word ‘table salt’ refers to a particular kind of salt in chemistry, the word ‘table sugar’ refers to a particular sugar in the world of chemistry. ‘Table sugar’ or sugar used at home is chemically known as sucrose and is a di-saccharide, which means two cyclic compounds, linked (through an oxygen atom) together containing four hydroxyl groups in each cyclic structure (one is 6-carbon chain and the other is a 5-carbon chain each with an oxygen instead of a carbon in its cyclic chain)instead of hydrogens. Some sugars like fructose are mono-saccharides containing the five chain cyclic poly-hydroxyl compound mentioned above and so is glucose with the same chemical formula as fructose(C6H12O6) but being the 6 carbon chain with one replaced by an O. In fact sucrose is manufactured with the starting mono-saccharides fructose and glucose. All sugars are sweet and are a store-house of energy, falling under the biological umbrella of carbohydrates.(Ref.4)
So far we have looked at examples of alicyclic compounds, but there is a vast number of cyclic compounds containing the benzene ring in them and are called aromatic compounds. Let us first look at the history and structure of benzene to understand these cyclic compounds.
Kekule, a German scientist pondered for days to figure out the structure of benzene whose chemical formula indicated six carbon and six hydrogen atoms. He kept thinking of straight-chain structures and was unable to arrive at any reasonable arrangement of the carbon and hydrogen atoms. Exhausted, he lay down to sleep and had a dream about a serpent eating its tail. Of course! Benzene was a ring structure with alternating double bonds that satisfied the chemical formula finally..(Ref.5)
We will continue the discussion of aromatic compounds, since the derivatives of benzene are vast and have great practical applications.
Activities for Middle School Teachers:
Students can once again create models of straight-chain alkanes, alkenes from carbon 1-10, 2-10 respectively. Then they can compare the structures and angles of C-C bonds with cyclo-alkanes starting of course with n=3, where ‘n’ refers to the number of carbon atoms and go up to n=10. Create a graph plotting number of carbon atoms to the angles between adjacent carbon atoms. Let students do a research project on actual angles seen with cyclo-alkanes having more than 5 carbon atoms. Are there stereo-isomers here as well?
Students can look at the chemical structure of cholesterol (Look up nuggets) and know what exactly ‘good’ and ‘bad’ cholesterol means.
Nuggets of information:
Cyclohexane is manufactured by the hydrogenation of benzene, a six carbon ring which is unsaturated. The nitration of cyclo-hexane leads to nylon products and its derivatives like balloons, ropes, etc.(Ref.1)
Sometimes, cyclic hydrocarbons are also called Naphthenes; these are the by-products of petroleum, usually cyclohexane and cyclopropane.(Ref.6)
Cholesterol, is another alicyclic compound with one of the six-membered carbon ring having an unsaturated bond. There are 3 parts to this compound: a hydroxyl group, a ring structure with 4 hydrocarbon rings and a hydrocarbon tail. The -OH or hydroxyl group is polar and soluble in water, so cholesterol is an alcohol. The 4-ring region lets us know it is a steroid hormone like estrogen or testosterone. All steroids begin with cholesterol. Since we have both the hydroxyl group and steroid part, it is called a sterol. The hydrocarbon tail is non-polar and is soluble in fats and not in water, making cholesterol not soluble in the blood stream.(Ref.7)
Brain scans reveal that sugar affects us in ways similar to drugs.(Ref.8)
Benzene is sometimes depicted as a hexagon with a circle inside it; this means that the electrons responsible for the alternating double bonds flow around like a ring of electrons.(More next time in Organic Chemistry)


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