In today’s lesson, we will cover two topics:
1. Properties of Alkanes, Cycloalkanes, Alkenes and Alkynes
2. Nomenclature of Cyclic and Aromatic Compounds, What is a conjugated system
Properties of Alkanes, Cycloalkanes, Alkenes and Alkynes
Alkanes and Cycloalkanes
Alkanes, and their respective ringed compounds cycloalkanes, are saturated hydrocarbons (which means they have no double or triple bonds – we discussed what alkanes are in a previous lesson. Click here for the link). At room temperature and atmospheric pressure (1 atm), these possess the following characteristics:
(a) Color: They are colorless
(b) Phases: They can be transparent gases, clear liquids or white solids, which depends on the molecular weight (aka molar mass)
(c) Odor: They can have a smelly odor
(d) Boiling Point Trends: The boiling point of a compound is when it transitions from a liquid to a gas. Boiling point increases as the number of carbons in the molecule increases (or when the molecular weight or molar mass increases). This is because the boiling point of a compound is a measure of how easy it is to break up the intermolecular attractive forces. Van Der Waals forces are a type of intermolecular force that stabilize the liquid phases of hydrocarbons like this. When two electron clouds (from two different atoms) approach one another, they induce dipoles (I’ll have a video up on the General Chemistry course page to explain dipoles more in depth).
For now, just know that many alkanes have small dipoles that do not really hold their molecules together tightly, so they are easy to break apart and therefore have a low boiling point. The longer or shorter/more compact a molecule is can also have an effect on boiling point. The longer a molecule is, the stronger the induced dipole can be, and more compact molecules have smaller induced dipoles and lower boiling points. Boiling point of straight chain alkanes is also more that branched chain alkanes.
For example, check out pentane and neopentane and their respective boiling points in the picture below. The more “compact” shape, neopentane, has a lower boiling point than the chain of 5 carbons, pentane.
(e) Melting point trends: The melting point of a compound is when it transitions from a solid to a liquid. Melting point increases as the number of carbons in the molecule increases (or when the molecular weight or molar mass increases).
Symmetry and compactness also both play a role in melting point. When we have a compact structure, surface area decreases and the shape is more “dense,” so melting point increases.
With regards to symmetry, highly symmetric molecules are are more “packed”, so they also have a higher melting point because they take more energy to break up.
For example, take a look at the picture below. Neopentane on the left is a symmetrical and more compact shape, whereas pentane on the right is symmetrical, but not as compact. The melting point of neopentane is more than 100 degrees C higher than that of pentane.
(e) Solubility: Alkanes are non-polar, so they are soluble in non-polar solvents (they are hydrophobic)
Alkenes
Characteristics of alkenes are same as those of alkanes, with a couple differences:
(a) Odor: A bit smellier than alkanes – the old name for alkenes, “olefins,” used to be named after its smell.
(b) Melting Point: Cis-alkenes do not pack as tightly so less energy is required to melt them as compared to trans-alkenes. Therefore, cis-alkenes have a lower melting point.
(c) Polarity: Alkenes are slightly more reactive due to the double bonds (aka pi bonds), so their dipole moments are more than that of alkanes. Trans alkenes dipole moments usually cancel out, whereas cis alkenes have a positive dipole moment
Alkynes
They have the same characteristics as alkanes, but here are some differences:
(a) Boiling Points: Slightly higher than alkenes due to a triple bond rather than a double bond.
(b) Odor and Color: They are all odorless and colorless, aside from ethylene which has a slight odor.
(c) Phase: Alkynes with up to 3 carbon chains are gases, 4 to 11 carbons are liquids, and 12 and higher carbons are solids.
Nomenclature of Cyclic and Aromatic Compounds
What is a cyclic compound? An aromatic one? A cyclic compound is any organic compound that is in a ring shape. Aromatic compounds, also known as “arenes,” are a special type of cyclic compound. When cyclic compounds exhibit something known as aromaticity, that means their rings consist of a conjugated system with delocalized pi-electron clouds rather than alternating double bonds. (I will have a lesson up under the Organic Chemistry II post to discuss Aromatics further).
Pi electrons are electrons making up a double bond, and delocalized pi electrons are those that are not bound to a specific bond or atom, but rather those electrons resonate between different atoms on the shape. A conjugated system is one in which 3 or more P-Orbitals are overlapping on adjacent atoms.
I show an example below of a comparison between a plain cyclic compound (a cycloalkane) versus a cyclic compound that exhibits aromaticity, aka an aromatic compound. Benzene on the right side is one of the most common aromatic compounds, so I will discuss how to name benzene as a main chain below.
A pi bond has 2 P-orbitals that overlap, like I show below.
We can have conjugated systems when we have (a) double bonds on adjacent carbons (b) A double bond next to a lone pair of electrons (c) A double bond next to a radical (d) A double bond next to a carbocation. I explain all these in the video linked near the top of the page.
How to name Cyclic Compounds vs Benzene as Main Chains
When we have a cyclic compound, it can be a substituent on a shape, OR it can be the main chain. When it’s a substituent, such as in the pictures on the left, we name it differently than if it were the main chain, such as on the right in the images below.
Let’s look at naming (a) Cycloalkanes and (b) Benzene as the main chain. We name all our substituents as we usually do (alkyl groups have higher priority over halogens, so I named the methyl group on carbon 1), and then we add “benzene” at the end of the name if it’s the main chain, like the picture at the top. The picture on the bottom is a cycloalkane, and it is a 6-membered ring – a cyclohexane. We have the same substituents, but we change the ending to “cyclohexane.”
Bicyclic Compounds
When two cyclic compounds become fused together, it can be done in different ways – see the three ways below (fused, bridged, or spiro).
Fused Bicyclic System
When carbons from both rings are directly connected (at the bridgehead positions), it becomes a fused bicyclic ring.
Bridged Bicyclic System
When both rings are connected with a “bridge” between them that consists of at least one carbon, it becomes a bridged bicyclic ring. We have the same shape drawn as a “top down view” on the left and a “side view” on the right. You can see this extra carbon I mentioned earlier pointing off the top of the shape in the right side image.
Spiro Bicyclic System
When two rings are both joined at the same carbon, it becomes a spiro fused molecule, as shown below.
Nomenclature and Examples
To name Fused and Bridged Bicyclic molecules, we use the same naming rules:
(a) Put “bicyclo” at the beginning of the name
(b) Put the name of the longest chain, as an alkane, at the end
(c) In the middle, use “[ ]” square brackets, which will have 3 numbers inside, indicating information about each ring, and the middle fused “bridgehead” carbons
Let’s take a look at an example using a fused ring:
Here we number around the two rings, counting all the carbons, which amounts to 10. A 10 carbon chain as an alkane is “decane,” so that is our suffix. We then put “bicyclo” at the beginning, and empty square brackets to indicate the name. This is what we get so far, leaving 3 empty spaces in the brackets:
bicyclo[_._._]decane
Then to fill in the 3 numbers that will go in the brackets, we have to indicate the number of carbons on the left ring (in blue), the right ring (in green) and the middle (in purple). See below:
There are 4 carbons in the left and right rings, and 0 in the middle. The two carbons fusing the rings together don’t count – if we did have a carbon in the middle, it would be one of those carbons in the bridged rings. Putting this together gives us bicyclo[4.4.0]decane.
Now let’s take a look at an example using a bridged ring:
Using the same rules, we count all the carbons in this ring, which amount to 8. Naming this as an alkane is “octane,” which will be the suffix. We put a
bicyclo” at the beginning, and leave square brackets in the middle. This gives us bicyclo[_._._]octane. Counting up the carbons on the left and right side, not including the “fused” carbons which I highlighted in yellow, give 2 carbons on both the left and right side of the bridge (carbons 5,6 and carbons 2,3) and 2 carbons in the center (carbons 7,8). This is bicyclo[2.2.2]octane.
Now let’s try naming a spiro bicyclic compound. For these, we put a “spiro” at the beginning of the name instead of a “bicyclo.” Then we add the name of the alkane, counting all the carbons, just like we did earlier with the other bicyclic compounds. We will leave 2 spaces in the square brackets, for the number of carbons on each ring, not including the common fused carbon. Let’s take a look at this example:
The total number of carbons is 8, which is an octane if named as an alkane. The number of carbons in the ring on the left is 4 (carbons 1,2,3,4), and on the right is 3 (carbons 5,6,7), not including the common fused carbon. This gives [4.3] in the brackets. Then we add a spiro at the beginning of the name. this gives spiro[4.3]octane.