Anyway, Organic Chemistry this year pretty much follows on from last year, but now we have some new functional groups to deal with: alcohols, aldehydes, ketones, carboxylic acids, esters and amines. I think I've got them all. It sounds like a lot, but it hopefully won't feel like a lot when you're actually learning it, because they're all somewhat related to each other (and not only by the fact that they're all organic compounds).
If you need a refresher from last year, check out these two blog posts:
- Basics of Organic Chemistry- covers alkanes, alkenes, alkynes, the cyclic compounds, aromatic compounds, basic reactions (combustion, addition, substitution) and naming very basic compounds.
- Naming and Drawing Organic Compounds- provides a worked example for naming a more complex organic compound, and another worked example for drawing an organic compound.
Right. Enough procrastination from me. Time to deal with our first functional group, which you're probably already reasonably familiar with already: the alcohols!
The alcohol that you're probably most familiar with would be ethanol, which is found in alcoholic drinks and is not to be confused with methanol. Its prefix eth- indicates that it has two carbon atoms, which it does. And, since it's an alcohol, it contains the alcohol functional group: an -OH group.
Methanol is pretty similar, but it only has one C atom and therefore less C atoms too. It also shares the -OH group, just like every other alcohol.
Alcohols have a special kind of secondary classification system, whereby you can classify them into primary, secondary or tertiary, depending on how many C atoms are attached to the same C atom as the -OH group. Yup, sounds confusing, but let me explain.
In the above picture, the -OH group is attached to the second carbon atom. This second carbon atom is only attached to one other carbon atom. Hence, ethanol is a primary alcohol. (I assume that methanol is also a primary alcohol, even though the C atom isn't attached to any other C atoms.)
Now for a secondary alcohol!
Meet propan-2-ol: prop- because it has 3 carbon atoms, -ol becase it has an alcohol group, and 2 because the alcohol group is attached to the second carbon on the chain. Here the -OH group is attached to carbon atom no. 2, which is attached to two others- no. 1 and no. 3- making this a secondary alcohol.
And now for a tertiary alcohol! (Don't worry, it won't go any further than this, since carbon can only form four bonds.)
This here is 2-methylpropan-2-ol. It's basically just propan-2-ol with a methyl group attached to the second carbon. As you can see, that poor little carbon in the middle is squashed by C atoms on 3 sides, its last bond being reserved exclusively for that OH group. Hence this is a tertiary alcohol.
Just a quick note on naming here: Naming for alcohols works pretty much the same way it has for the previous types of hydrocarbons covered so far. The main difference is that you have to add -ol on the end this time. Also, when numbering, the -OH is always given its lowest possible number, just like how the double bond of an alkene was always given the lowest possible number. For example, if you have a molecule with 8 carbons, and you have two chlorine atoms attached to the second and third carbons and the -OH group attached to the seventh, then you'd end up with 6,7-dichlorooctan-2-ol, NOT 2,3-dichlorooctan-7-ol. In other words, the functional group ALWAYS takes precedence over the other stuff when it comes to assigning numbers.
Now, what happens if you get, say, a double bond and an -OH group? I don't know. I think that one takes precedence over the other, so one becomes the suffix and the other just gets delegated to being another random thing attached to the main chain. Actually, I think in this case the double bond wins out, so the molecule name would have an -ene ending and then the -OH group becomes hydroxy- or something to that effect.
Anyway. Back to the different kinds of alcohols. You're probably asking the number one question at this point: WHY DOES THIS MATTER? Indeed, why does it matter?
It matters because the three different types of alcohols can undergo different reactions (or, rather, the same reaction) to produce different types of chemicals. Apart from combustion, and possibly substitution, it's also possible to oxidise alcohols. Well, in any case, it's possible to oxidise primary and secondary alcohols- tertiary alcohols cannot react in this way.
One of the definitions of oxidation is the addition of oxygen. Another definition is the removal of hydrogen. It's the latter definition that's important here. In a primary alcohol, the carbon that the -OH is attached to loses one hydrogen. In addition, the H on the -OH is removed. This paves the way for an =O group to form, and a new organic compound known as an aldehyde. This is ethanal, which you get initially when ethanol oxidises:
Note that I said initially. I said this because, if the reaction is allowed to continue (i.e. you don't distill off or somehow collect the aldehyde as soon as it's formed), the aldehyde then becomes a carboxylic acid via the addition of oxygen:
The above molecule is ethanoic acid. Does it sound familiar? It should, if you've been studying chemistry all these years. It's also known as acetic acid, and if you get rid of the H on the -OH group and turn it into a negative ion, then it becomes the ethanoate ion. Ethanoic acid is also found in vinegar and is responsible for its smell (I think...).
Naming aldehydes and carboxylic acids is the same as naming any other molecule thus far, except that aldehydes end in "-al" and carboxylic acids end in "-oic acid." If you have trouble remembering this, just remember that aldehydes start with "al" and that our good old friend ethanoic acid is a carboxylic acid that ends in "-oic acid." Also, you don't have to worry about providing a number for the aldehyde or carboxylic acid group, since these groups are always on the end of a carbon chain due to having being derived from a primary alcohol.
I can't be bothered writing sample reactions for these, since they're the same as redox reactions (go read my post on Redox Equations if you need to brush up on this). Permanganate and dichromate solutions are still pretty common here. Sometimes heat is needed too.
Secondary alcohols can undergo a similar reaction to produce a ketone, which is like an aldehyde but the =O group's not on the end. It's given a different name because ketones have different properties and undergo different reactions. For one thing, they cannot undergo further oxidation.
This is propan-2-one, or propanone for short (remember, ketone groups cannot be on the ends of molecules, otherwise they'd be aldehydes):
Again, naming conventions are the same, but this time names end in -one.
I'm wondering whether to talk next about the reactions of alcohols with reactive metals or to talk about the formation of esters. Eh, I'll talk about reactive metals.
Y'know how, when water reacts with an alkali (Group I) metal, there's a really violent reaction in which the metal's consumed? No? Then watch this video:
In these reactions, a metal dissolves in water to produce the metal ion, hydroxide ions and hydrogen gas.
K (s) + 2H2O (l) à K+(aq)
+ 2OH-(aq) + H2 (g)
Now let's rewrite the equations in a slightly different manner in order to pave the way to understanding how this applies to alcohols:
K (s) + 2H-OH (l) à K+(aq)
+ 2H-O-(aq) + H2 (g)
Now let’s consider how alcohols can be represented by the
general formula R-OH, where R is a hydrocarbon chain, and substitute this into
the equation:
K (s) + 2R-OH (l) à K+(aq) + 2R-O-(aq) + H2 (g)
Hence, the reaction of an alcohol with an alkali metal results in a metal ion, an alkoxide ion (names end in -oxide, e.g. ethoxide ion, pentoxide ion etc.) and hydrogen gas.
Now back to esterification!
Esterification is the production of esters. Esters are formed when alcohols and carboxylic acids are mixed under the presence of a catalyst such as sulfuric acid. They often smell quite fruity and, indeed, they're often found in fruits. When the alcohol and carboxylic acid react, one loses an H atom while the other loses its -OH group. I can't remember which one loses which, but it's not important at this stage.
After losing all these atoms, the compounds then join together to form an ester:
Meet methyl ethanoate. (I chose this one because it has relatively few atoms but has two different chain lengths.) This ester was formed from the reaction between methanol and ethanoic acid. The methyl part comes from methanol and the ethanoate part comes from ethanoic acid. In fact it's kinda like a methyl group joining to an ethanoate ion, but not. Or maybe it is? I don't know.
All ester names are two words long. The part that is derived from the alcohol is given the -yl ending while the part that is derived from the carboxylic acid is given the -oate ending, just like the negative carboxylate ions of these acids.
Wait. I haven't talked about carboxylate ions yet. Silly me. Those carboxylic acids are called carboxylic acids for a reason- they can react as acids, losing a proton (i.e. an H+ ion) in order to form a conjugate base. Carboxylate ions are essentially the conjugate bases of carboxylic acids- they're carboxylic acids with one less H and a -1 charge. Naming follows normal conventions but end in "-oate ion."
Another way to remember which part has the -yl and which part has the -oate is that -oate is longer and the -oate side also has an =O group, or, to be kinda crass (I think I used that word correctly...?), the side that has more crap on it gets the longer name.
I'm not sure how to name esters if they have branches. I'm not sure if I've really encountered any in my textbooks. There was one that I did remember encountering but the question even specifically said that naming it was "beyond the scope of this course" or something like that.
Last but not least, I'm finally up to talking about the last functional group covered in the course: the amines! Amines are pretty simple. Basically, they're any compound with the -NH2 group. Like alcohols, they are also classified into primary and secondary, but, unlike alcohols, the classification works differently. I'm not sure how the classification system here works, because we've only covered primary amines in class. Let's just say that most if not all of the amines you're gonna see this year will be primary amines.
Amino acids, those things that you hear about in biology, are a bit different. They're amino acids, you see, so they have an amine group and an acid group! How exciting. w00t. Now, one thing really funky about amino acids is that the -NH2 amine end is capable of forming a positive -NH3+ ion, while the -COOH carboxylic acid end is capable of forming a negative -COO- ion... AND BOTH ENDS CAN DO THIS AT THE SAME TIME!! Yes, you can have an ion that is both positive AND negative! This is called a zwitterion (pronounced "zwitter ion"). As zwitterions, amino acids can form ionic compounds (the positive end of one bonds with the negative ion of another) and thus take up many characteristics of ionic compounds such as high melting and boiling point.
Oh and having both a basic -NH2 end and an acidic -COOH end means that amino acids are not just zwitterions, they are also amphoteric (acidic or basic depending on the situation)! It'll become an acid in a basic solution by losing a hydrogen ion on its carboxylic acid end (leaving behind a -COO- ion- the -NH2 group stays neutral), or a base in an acidic solution by gaining a hydrogen ion on the amine end (resulting in a -NH3+ ion- the -COOH group stays neutral). In a neutral solution, the amino acids take up their zwitterion form. This amphoteric nature of amino acids makes them good buffers.
One last bit of terminology for you- if the -COOH group and the -NH2 group are attached to the same carbon atom, and that carbon atom is at the end of the chain, the amino acid is called an alpha-amino acid. (Replace "alpha" with the Greek letter.)
That's pretty much it from me. Yay! I've finally gotten this post out to the world after having it sit around in draft form for the past five months or so! If you want to know a bit more about how organic chemistry relates to human bio, you can check out my new post at http://year11misadventures.blogspot.com.au/2014/08/diffusion-osmosis-enzymes-and-organic.html. Have a nice day! :D
Oh and having both a basic -NH2 end and an acidic -COOH end means that amino acids are not just zwitterions, they are also amphoteric (acidic or basic depending on the situation)! It'll become an acid in a basic solution by losing a hydrogen ion on its carboxylic acid end (leaving behind a -COO- ion- the -NH2 group stays neutral), or a base in an acidic solution by gaining a hydrogen ion on the amine end (resulting in a -NH3+ ion- the -COOH group stays neutral). In a neutral solution, the amino acids take up their zwitterion form. This amphoteric nature of amino acids makes them good buffers.
One last bit of terminology for you- if the -COOH group and the -NH2 group are attached to the same carbon atom, and that carbon atom is at the end of the chain, the amino acid is called an alpha-amino acid. (Replace "alpha" with the Greek letter.)
That's pretty much it from me. Yay! I've finally gotten this post out to the world after having it sit around in draft form for the past five months or so! If you want to know a bit more about how organic chemistry relates to human bio, you can check out my new post at http://year11misadventures.blogspot.com.au/2014/08/diffusion-osmosis-enzymes-and-organic.html. Have a nice day! :D
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