Chemistry

Chemistry

octane – unbranched (straight-chain)

4-ethyl-2-methyloctane – branched

ethylcyclohexane – cyclic

Figure 1 – Unbranched, branched, and cyclic hydrocarbons.

Experiment #3 – Hydrocarbons

Introduction

Organic chemistry is the chemistry of the compounds of carbon. Currently over twenty million compounds have been reported in the chemical literature; about 90% of them are organic, ie they contain carbon. The remaining compounds are called inorganic and are formed from the other elements, of which there are about 100. That carbon so dominates compound formation is a result of the fact that it is almost unique in its ability to form long chains with other carbon atoms. [Carbon’s neighbor in the periodic table, silicon, can do this but rarely does.] These chains with one carbon joined to a second and the second joined to a third, etc., can be branched, ie, chains of carbon atoms can be attached to carbons in the original chain. It is also possible for one carbon in a chain to become bonded to another carbon in that chain, resulting in a closed ring of atoms. We call these compounds cyclic.

Since there are so many organic compounds it is fortunate that we can organize them into various groups that have some similarity to each other. For example, one large group of organic compounds is known as the hydrocarbons because members of this group contain only carbon and hydrogen and no other elements. Figure 1 shows examples of branched, unbranched and cyclic hydrocarbons.

It is possible to subdivide the hydrocarbon group of compounds based on the bonding between the carbons. If all the carbon-carbon bonds are single, the compound is an alkane. If at least one of the carbon-carbon bonds in the compound is a double bond, and the remaining carbon-carbon bonds are single, the compound is an alkene. If at least one of the carbon-carbon bonds in the compound is a triple bond, and the remaining carbon-carbon bonds are single, the compound is an alkyne. If the compound contains a six carbon ring that has alternating double and single bonds around the ring (three double and three single), we say that ring, and the compound, is aromatic. An aromatic ring looks like an alkene with three double bonds because that’s the way we draw it using Lewis structures. However, the actual bonding in such a ring is considerably different from that in alkenes and, consequently, many of the chemical properties are different also. Therefore, we place these compounds in a separate family. By the way, the term aromatic as used here has nothing to do with fragrance.

Experiment #3 Hydrocarbons Page 2

Hydrocarbons may be saturated or unsaturated. A saturated hydrocarbon is one that is maxed out in terms of the number of hydrogens that can be present given the number of carbons in the compound; it is impossible to add more hydrogen atoms to the compound so it is saturated with hydrogen. Acyclic (no rings) alkanes are saturated; there is no way additional hydrogens can be added while keeping the same number of carbons and maintaining normal bonding between the atoms. Alkenes, alkynes, aromatic compounds, and cyclic alkanes are unsaturated because hydrogen can be added to them, in theory and usually in practice, making them into acyclic alkanes. Some examples follow.

Physical Properties

Some molecules carry an electrical charge because there is a difference between the number of electrons (each with a -1 charge) and the number of protons (each with a +1 charge) in the molecule. We call molecules of this type ions. The ammonium ion, NH4

+ , has a +1 charge because it has 11 protons (7 from nitrogen and 4 from the

hydrogens) and 10 electrons (2 in nitrogen’s first shell, and 8 in its second shell [which are used to bond the hydrogens to the nitrogen]). So, with 11 plus charges and 10 minus charges, the ammonium ion has a net charge of +1. Most molecules are not ions (and we simply call them molecules); in other words they are electrically neutral – they have a charge of 0 because they have the same number of protons as electrons.

H3C CH CH CH3 H2 H3C CH CH CH3

H H

+ catalyst

H3C C C CH3 catalyst+ H3C C C CH3

H H

H H

H22

C C

C

CC

C H

HH

H

H H

catalystH23

C C

C

CC

C H

HH

H

H H

H H

H

H

H

H

+ H2 in theory,

if not easily done in practice

C C

C

CC

C H

HH

H

H H

H H

H

H

H

H H H

unsaturated compounds

saturated compounds

Experiment #3 Hydrocarbons Page 3

φ

φ

φ

φ

v

v v

m

m m n = ss =

sin sin

vacuum

materiallight beam

light beam

Figure 2 – Index of Refraction

In some molecules, even though the net charge is 0, the distribution of positive charges (protons) and negative charges (electrons) within the molecule is not the same. Such a molecule has a lopsided charge distribution – one side of the molecule is electron rich (has a partial negative charge) and the other side is proton rich (has a partial positive charge). We say that such a molecule is polar or that it has a dipole moment. The more lopsided the charge distribution, the larger the dipole moment and the more polar the molecule. In some molecules the distribution of positive and negative charges is the same; these molecules have no dipole moment and are nonpolar. When organic molecules are polar it is usually because there are one or more highly electronegative atoms on one side of the molecule. Electronegative atoms are those that attract electrons toward themselves; common examples are nitrogen, oxygen, and the halogens, especially fluorine and chlorine.

Polar molecules attract each other because the negative side of one attracts the postive side of another (opposite charges attract). The more polar the molecules the more they attract each other. It is also true that nonpolar molecules do not attract each other as strongly as polar molecules do and polar molecules are not attracted to nonpolar molecules as much as polar molecules are attracted to each other.

Hydrocarbons are molecules that have little or no polarity because they do not contain electronegative atoms. They are soluble in solvents of low polarity. They are not soluble in water, which is very polar, because the water molecules attract each other strongly (and are not nearly as interested in attracting nonpolar molecules).

Density is the mass of a material divided by its volume; it is often expressed in terms of grams of mass per cubic centimeter of volume. The density of water, for example, is 1.00 gram per cubic centimeter. For the most part, hydrocarbons are less dense than water, so, given their insolubility in water, they float on it. Crude oil and its derivatives gasoline, kerosine, and fuel oil are mainly hydrocarbons; none of these is soluble in water and they float on its surface since they are less dense than water.

The index of refraction, n, of a compound is the speed of light in a vacuum, sv divided by the speed of light in that substance, sm. Since light travels faster in a vacuum than anywhere else, the index of refraction is greater than 1.000 for any substance. It turns out that the index of refraction can be measured by noting how much a beam of light bends when it travels from a vacuum (or air, since the speed of light in air is almost the same as that in a vacuum) into some substance. See Figure 1. The index of refraction of liquids is usually measured using an Abbé refractometer where the liquid is placed on a glass prism of known index of refraction. [Glass is easier to work with than a vacuum.] Because of the large number of electrons located around the ring in aromatic compounds these compounds usually have larger indexes of refraction than other hydrocarbons. Typically, aromatic compounds will have indexes of refraction greater than 1.45, while other hydrocarbons will have smaller values.

Experiment #3 Hydrocarbons Page 4

C3H8 O2 CO2 H2O5 3 4+ +

H3C CH CH CH3 H3C CH CH CH3

Br Br

+

H3C C C CH3 + H3C C C CH3

Br Br

Br Br

2

Br2

Br2

Natural gas is composed of alkanes; it is about 90% methane with small amounts of ethane and propane. Methane, ethane, propane, and butane are gasses at room temperature. Pentane is barely a liquid at room temperature and the higher molecular weight alkanes are liquids, the straight-chain versions becoming solids at about 16 carbon atoms. Butane boils at about 0

o C, which is why butane lighters do not function well

below that temperature: the liquid changes to gas too slowly below its boiling point to maintain a flame.

Chemical Reactions of Hydrocarbons

1. Combustion.

All of the hydrocarbons undergo combustion – they burn in the presence of oxygen. If there is enough oxygen the combustion will be complete, ie, the products of the combustion will not burn. In fact, if combustion is complete the products will be carbon dioxide and water, and, of course, heat is given off – the reaction is highly exothermic – which is ordinarily the purpose of this reaction. The equation for combustion of propane (bottle or LP gas, used for cooking and heating, is mainly propane) is shown below.

2. Reaction with Bromine.

Hydrocarbons with multiple bonds (unsaturated hydrocarbons except most cycloalkanes) react with bromine. Tetrachloromethane (carbon tetrachloride) or cyclohexane are usually used as solvents because they are unreactive toward both bromine and hydrocarbons that have multiple bonds.

Alkenes and alkynes undergo an addition reaction with bromine. The double bond of an alkene becomes a single bond and one bromine atom becomes bonded to each of the carbons that had shared the double bond. No other product is formed; the alkene and bromine simply add together, which is why it’s called an addition reaction. The triple bond of an alkyne also undergoes an addition becoming a single bond, but in this case each of the carbons that had been joined by the triple bond will now hold two bromine atoms. Examples follow.

This reaction usually occurs quickly at room temperature without a catalyst.

Experiment #3 Hydrocarbons Page 5

C C

C

CC

C H

HH

H

H H

H H

H

H

H

H

+

C C

C

CC

C Br

HH

H

H H

H H

H

H

H

H

Br2+ HBr heat or

ultraviolet light

aromatic + alkene

aromatic + alkane

aromatic only

(FeBr3) H BrBr2 HBr+ +

catalyst

CH2 CH3 + Br2

CH CH2 Br2+ CH CH2

Br Br

addition reaction to alkene double bond, bromine color dissipates quickly

reaction quite slow in the absence of catalyst, bromine color fades quite slowly

reaction slow in the absence of catalyst, bromine color fades slowly

CH2 CH3

Br

CH2 CH3Br

+ catalyst

(FeBr3)

Bromine is a reddish-brown color. All of the other substances in these reactions are colorless. So, when bromine is added to an alkene or alkyne the red-brown color dissipates quickly, often almost instantly.

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