Alkanes and Alkenes
Written by tutor Nathan R.
Dealing with organic compounds in chemistry can feel overwhelming. A small difference in the molecule changes the name and can drastically change the reactivity of the compound. This holds true for the two compound groups, Alkanes and Alkenes. They are both hydrocarbons and therefore follow the standard nomenclature prefix (ex. meth, eth, prop, but, pent). Their suffix differs based on the type of carbon bonds present. The suffix difference is one of many differences that will be discussed further.
As stated earlier the beginning of a compounds name (meth, eth, and prop) stays the same while the ending changes based on the type carbon bond. There are three types of carbon bounds; single, double and triple. The focus will be on the carbon to carbon single and double bonds or the alkanes and alkenes, respectively. A two carbon chain with a single carbon to carbon bond would be named ethane while a two carbon chain with a double carbon to carbon bond would be named ethene. That same naming scheme applies to the larger carbon chains as well (octane, octene, etc).
Single bond -> H3C-CH3 -> ethane
Double bond -> H2C=CH2 -> ethene*
*Notice above that ethene has 2 less total hydrogen atoms
What is meant by a single and double carbon to carbon bond is the number of paired covalently bonded electrons present. So a single carbon to carbon bond has 1 pair of electrons (2 electrons) bonding them together while a double has 2 pairs of electrons (4 electrons) covalently bonded. The alkane is saturated while the alkene is unsaturated.
All hydrocarbons are saturated or unsaturated. Saturated hydrocarbons have only single carbon to carbon bonds. Like soil that is saturated with water and no more water can be absorbed into it. Saturated hydrocarbons will not accept anymore hydrogen atoms. Alkanes follow this molecular formula:
So ethane, which has 2 carbon atoms, would have [2 x 2(# of C)] + 2 = 6 or the number of hydrogen atoms in ethane.
Unsaturated hydrocarbons, by definition, must have at the very least a double, also known as a pi bond. Alkenes are more reactive than alkanes due to the double bond(s). The molecular formula for alkenes is very similar to the formula for alkanes. It differs in this way. For every double bond present you subtract 2 from the above calculation. Example is as follows:
Ethene has 1 double bond so the calculation becomes [2 x 2(# of C) + 2]-2 = 4 hydrogen atoms
So now you know the difference between saturated and unsaturated fats!
Neither ethane nor ethene requires much in the way of rules to name them. Either the carbon bond is single or it is double, ethene or ethane. When one attempts to name larger alkenes the issue of what carbon bond contains the double bond becomes apparent. A person should be able to draw the structure of a molecule by its name and name the molecule by its structure. One must always lead to other and be reversible to be correct. Without numbering the carbon atoms it will remain unknown which carbon(s) contain the double bond(s). So the pi bond is assigned a number based on which carbon atom in a hydrocarbon chain it begins. For example let us look at a hexene below.
When counting carbons to determine the name of a generic alkane it does not matter if one counts from left to right or right to left. But, when counting carbons to name an alkene, the direction one counts in does matter. As in the image above counting from the left to the right has the pi bond starting at the second carbon. When counting from right to left the pi bond begins at the fourth carbon. So 2-hexene or 4- hexane, which is it? The rule is to count in the direction that will result in the pi bond starting at the smallest numbered carbon possible. So 2-hexene is correct!
One more note about naming alkenes. When the pi bond starts at the first carbon it is just named hexene. Why not 1- hexene? Similarly to 1x = x in algebra, 1- hexene = hexene.
Reactivity of alkanes and alkenes are very different. In that alkanes very rarely react while alkenes are highly reactive. Alkenes readily react with halides, alcohols, and hydrogen to name a few. Alkene bromination, or halogen addition reaction, is one of the more well know reactions because it is used to determine if an organic molecule is saturated or unsaturated. Take the 2-hexene, as shown previously, and then mix it with Br2, that gives us the following as a product: 2,3- bromohexane. Notice the ending is back to the alkane ending due to its loss of the double bond.
Hydration of the alkene starts off with water and some sulfuric acid acting as a catalyst. The pi bond is broken and the H and the OH from the H2O take its place. 3- Hexanol is shown in the image below.
These are just two of the more common reactions for alkenes. Feel free to contact me if you have any other questions.
Alkanes and Alkenes Review
So did you get everything?
For an added challenge, try naming the following compound: