Organic Lecture 1: The Effects of Hydrocarbon Branching on MP and BP

Let me preface our first organic lecture with a brief excerpt on how to succeed in studying for this portion of the test. My advice is to approach studying organic like you would approach studying a foreign language. It's a mistake to believe that you can simply memorize this material and perform well on any organic chemistry test, especially the MCAT. There are an infinite number of possible reactions out there so you first need to understand the vocabulary (mechanisms) and grammar of organic. How does one accomplish this? Work lots of problems and seek help when needed. With this said, I understand that by reading this post most of you have already taken at least one semester of the course. If you have and did well, congratulations. It's a class that has ruined more medical careers than any other. Studying for MCAT organic will luckily be a little more mundane than the preparation you put forth for the actual class. Flash cards won't be necessary and there are no synthesis problems on the MCAT--although they do find a way to test these concepts indirectly as we will soon discover. So without further adieu, let's begin our first organic lesson with melting/boiling points and how hydrocarbon branching affects each.

For starters, melting point (mp) and boiling point (bp) are indicators of how well identical molecules interact with (attract) each other. Nonpolar molecules interact principally due to the London dispersion force, one of the intermolecular forces. Such forces can be overcome to melt a nonpolar compound or to boil a nonpolar compound. The greater the attractive force between molecules, the more energy will be required to get the compound to melt or boil. Branching is the most significant factor in determining the degree to which molecules will interact. Branching tends to inhibit van der Waals forces by reducing the surface area available for intermolecular interaction. Thus, branching tends to reduce attractive forces between molecules and to lower both melting points and boiling points.

Another influencing melting point and boiling point for hydrocarbons is molecular weight. The greater the molecular weight, the more surface area there is to interact, the greater number of van der Waals interactions, and the higher the melting point and boiling point.

So, to get to the meat of the lesson, what is the effect of hydrocarbon branching on mp and bp?

Here's the short answer: Branching will decrease mp and bp. Condensing and freezing happen for alkanes because of dispersion forces caused by temporary, induced dipoles, which are the only intermolecular forces holding nonpolar molecules like alkanes (hydrocarbons) together in the liquid and solid states. Branched alkanes have smaller dispersion forces compared to straight-chain alkanes of the same MW, so they will be harder to liquify or freeze.

Here's the long story: Let's imagine that we are cooling down a gaseous alkane like hexane that is straight-chained, versus another of the same MW that is branched, like 2,3-dimethylbutane. Remember that gases don't have any intermolecular interactions, at least not if they're ideal. As we lower the temperature, the molecules stop moving as much, and they begin to have intermolecular interactions that are due to induced, temporary dipoles called London dispersion forces.

Ok, so now we need to consider what kinds of molecules will have the strongest induced dipoles. The strength of the induced dipoles is directly proportional to the surface areas of the molecules that are coming into contact. This is intuitive, because if contact can be made over a greater area, there is a greater chance that electrons will distribute unequally at some point over that surface, causing the temporary dipole and inducing dipoles in the neighboring molecule. You may know that the shape with the smallest surface area-to-volume ratio is a sphere. So molecules that are more spherical (ie, highly branched) do not have very much surface area relative to molecules that are long and extended (straight chains). That is why branched molecules have weaker dispersion forces versus straight-chains. Since they have weaker dispersion forces, the branched molecules will tend to want to stay in the gaseous phase longer, and you'll have to cool them further to force them to condense into a liquid. This means that branched compounds have a lower bp (condense at a lower temperature) versus straight chains.

As we continue to cool, the molecules continue to move closer and closer together, and their interactions continue to increase. Eventually, we reach a point where they begin to crystallize, or at least form an amorphous solid. So we need to consider how well the molecules pack together at this point. Branched compounds are like little spheres, or like porcupines. It's hard to get them to pack well, and this means that you will have to cool them to a lower temperature to freeze them (lower mp) compared with straight chains, which can stack up nicely, more like a cord of firewood. So the mp of a branched compound will be lower than that of a straight chain, assuming that they have the same MW.

Sorry for the shoddy image. You can click on it and it will get bigger if it appears too small.