FreeMCATPrep Logo

Support us and cryptocurrency!
Try a browser that's faster, safer, ad-free, and earns you cryptocurrency for using it! W3Schools

More Than 475 Free MCAT Questions with Detailed Answers!

Click HERE for your Random Question from our MCAT Question A Day Archive

Organic Chemistry - Alkanes and Alkenes

If all of the carbon atoms in a molecule are linked together by single bonds, the molecule is called an alkane. An alkene means a hydrocarbon chain in which at least two of the carbons are connected by a double bond. 

In order to name alkanes or alkenes, we count the carbons in the carbon chain. If the number of carbons is one, you say meth-; if it is two, you say eth-; three, you say prop-; four, you say but-, five, you say pent-; six, you say hex; seven you say hept-; eight you say oct-; nine, you say non-; ten, you say dec-. Then you add -ane for an alkane and ene for an alkene. If the molecule is an alkene, put a number in front of it. The number in front of an alkene’s name indicates the carbon in the molecule where the double bond begins. To find this number, you count the carbons starting from the end nearest the double bond. Now if for example a molecule is called 2-methylheptane; that means that on the second carbon of a heptane there is a methyl group attached. For alkenes that have another group attached, we have to make the name a little more descriptive. So if there is methyl group stock on carbon number 2 of a 3-octane, we call this molecule 2 methyl, 3-octene. Remember that when you are counting the length of a carbon chain you must count the longest continuous chain even if is not straight across the page. A carbon chain can be branched, having all kinds of branches attached to it. The branches might be long or short assuming all kinds of shapes and configurations. You should know about three special kinds of branches and be able to recognize them. 

An iso group is a branch that ends with the carbon bonded to two methyl groups and a hydrogen atom. The smallest possible iso group is an isopropyl group. So, if we attach a chlorine to the fourth side of an isopropyl group, we would have isopropyl chloride and if we attach another carbon atom, we would have an isobutyl group. A tert-butyl group is a branch that ends with the carbon bonded to three methyl groups. The smallest possible tert-group is a tert-butyl group. So, if attach an OH group to the fourth side of the group, we would have tert-butyl-alcohol. A sec-butyl group is a branch that ends with the carbon attach to a hydrogen on one side, a methyl group on another and the methyl group on the third. The smallest possible sec- group is a sec-butyl group. If on the fourth side, we would attach a fluorine atom; we would have sec-butyl fluoride. 


Any molecule’s solubility in things depends a lot on whether it is a polar molecule or a non-polar molecule. Remember that when it comes to polarity and non-polarity, like dissolves like. So, polar molecules are soluble in other polar molecules and non-polar molecules are soluble in other non-polar molecules. Alkanes and alkenes are non-polar molecules. Water on the other hand is a polar molecule. So, alkanes and alkenes are not soluble in water. 

Boiling and Melting Point 

For alkanes and alkenes, higher molecular weight tends to mean higher melting points and boiling points. Since the boiling points of alkanes and alkenes tend to be higher when molecular weight is higher, it turns out that the shorter molecules are gases at room temperature and the larger ones tend to be liquids in room temperature. Very big alkanes and alkenes with chains that have more than 17 carbons on them are solids at room temperature. Now there is one more thing you should know about boiling points of alkanes and alkenes; it has to do with branching. 

If two molecules have the same number of carbons so their molecular weights are about the same but one molecule is a straight chain and the other is a branched chain. The branched molecule will have a lower boiling point than the straight chain molecule. So, the two things to remember are one, increasing the molecular weight of an alkane or alkene carbon chain increases its melting and boiling points and two, branching tends to lower the boiling point. 

Now that we have talk a little about how molecular weight and branching affect the properties of alkanes and alkenes, let us talk some about what alkanes and alkenes do. Let us start with things that alkanes do. Alkanes undergo a reaction called a halogenation and you should know a little about this halogenation reaction. This is what happens in halogenation. An alkane meets up with the molecule of a halogen like fluorine or bromine and one of the alkanes’ hydrogens get substituted by one of the halogen atoms. The halogenation of an alkane is a chain reaction. There are three steps involved in this chain reaction. Step 1 or chain initiating step is simple. In this step, a halogen molecule is split in to two atoms under the influence of ultraviolet light. Now each of the 2 atoms has an unpaired electron. 

At step 2A, the alkane comes on the scene. Let us say that in this case the alkane is methane and the halogen is bromine. One of the bromine atoms robs a hydrogen atom off of the alkane and forms a molecule of hydrogen bromide. That means the alkane is missing a hydrogen. It also leaves the alkane with an unpaired electron. The little dot next to the CH3 indicates that this robbed alkane molecule has an unpaired electron. The robbed alkane molecule is called an alkyl free radical. Remember that these alkyl free radicals are not stable. They are looking for something to react with. In this case, the alkyl free radical reacts with another molecule of halogen to form CH3Cl. 

At the end step of 2B, we are left with our halogenated alkane and a single halogen atom. This single halogen atom can now react with another molecule of alkane and thereby repeat step 2A. Once step 2A has been repeated, step 2B can be repeated then step 2A can be repeated again and step 2B can be repeated again and so on and so on. Eventually the reaction terminates when a radical reacts with another radical. If a carbon is attached to only one other carbon, we call them primary carbons. If a carbon is attached to two other carbons, we call it a secondary carbon. If a carbon is attached to three other carbons, it is a tertiary carbon. In forming a free radical, the hydrogen is more likely to be taken from the carbon that will leave the most stable free radical. 

So, the hydrogen is more likely to be taken from a tertiary carbon than from a secondary carbon and it is more likely to be taken from a secondary carbon than from a primary carbon. It is important to remember with respect to alkyl free radicals, the order of stability which is tertiary more than secondary, more than primary. Another way of saying this is it takes less energy to form a tertiary alkyl radical than it does to form a secondary alkyl radical and it takes less energy to form a secondary alkyl radical than it takes to form a primary alkyl radical. If enough heat is supplied for activation energy alkanes can burn or combust. That means they react with oxygen to form carbon dioxide and water. 

So, there are two reactions alkanes can undergo; halogenation by UV light and combustion. Sometimes, alkanes will form rings. You need to remember that the angles in the ring produce something called angle strain. Angle strained tends to make a ring unstable. The more angle strain, the less stable the ring. The less angle strain, the more stable the ring. When we are talking about rings with fewer than seven carbons, the simple rule is the more carbons in the ring, the less the angle strain and of course the less the angel strain, the more stable the ring. Cyclohexane is an important ring that you should know about. Confirmations are the different spatial configurations of a molecule. You should know 2 confirmations for cyclohexane, chair and boat. The chair confirmation is more stable than the boat form. The way to represent confirmations is by drawing Newman projections. Remember that when the methyl groups in a molecule are closer together, they produce greater esteric strain. In the case of alkenes, we are dealing with double bonds which cannot rotate. So, if there are two molecules attached to the alkene on the same side of the double bind and two molecules attached on opposite sides of the double bond, we have two different alkenes even though they may have the same formula. The alkene with the molecules on the same side of the double bond is called the cys-isomer of the alkene. The alkene with the molecules on the opposite sides of the double bond is called the trans-isomer of the alkene. So, the point is this, if you are looking at an alkene and the two double bounded carbons have 2 like constituents attached to them, what you call them depends on where the constituents are attached. If the light constituents are on the same side you say cys-isomer. If the light constituents are on the opposite sides you say trans-isomer. Remember that a cys-isomer is a dipole. A cys-isomer is a dipole because charge is not symmetrically distributed around the carbon-carbon double bond. If you think about the upper and lower sides of the double bond, you will realize that one side is going to be a little positive and one side is going to be a little negative. Trans-isomers on the other hand are not dipoles. That is because the polar bonds are arranged so that the centers of positive and negative charge are in the same place. You need to know that when we are dealing with alkenes, we can get rid off of the double bond by adding things to the double-bonded carbons. The thing that is usually added is a hydrogen halide like hydrogen chloride, hydrogen bromide or hydrogen iodide and the reaction is known as electrophilic addition. This reaction is called an electrophonic addition because the hydrogen is added to the double bond first.