Organic Chemistry Structure and Nomenclature

What is Organic Chemistry This is the branch of chemistry in which the focus is on the compounds that are composed (primarily) of carbon and hydrogen. It is possible (and common) for there to be other elements in these substances, but the majority of the atoms are C and H.

Are There Classes of Organic Compounds? Yes – just as we classify compounds in general chemistry based on what elements and behaviors are observed (ionic, covalent, acids, bases, hydrates, etc)… We classify organic compounds based on the elements present and the bonding and structure. Alkanes Alkenes Amines Alcohols etc

Where do we Start? The easiest of the organic compounds are the Alkanes. They consist of only carbon and hydrogen and only have single bonds between the atoms. Things to remember: – Carbon will form 4 bonds – It will use sp 3 hybrids – The bond angles will be 109.5° – The hydrogen atoms will always be on the “outside”.

Alkanes The are the organic compounds that have the generic formula C n H (2n+2) Examples are: CH 4 C 2 H 6 C 3 H 8 C 4 H 10 C 5 H 12

Naming Alkanes In organic, the name of the compound typically starts with a prefix that represents the number of Carbon atoms in the “backbone” of the structure. C=meth C 2 =eth C 3 =prop C 4 =but C 5 =pent In the alkanes, all you have to do is add the suffix “ane” to let the reader know that you have an alkane.

So…… CH 4 C 2 H 6 C 3 H 8 C 4 H 10 C 5 H 12 =Methane =Ethane =Propane =Butane =Pentane

Consider the Structures Cambridge will want you to be able to represent a given organic molecule three different ways. Structural or “Lewis Dot” (perhaps dot and cross). Displayed Skeleton

Examples

Isomers The word “isomer” actually translates to “same units”. – Iso means “same” – mer is from the Greek for “unit” These are different compounds that have the same chemical formula. The different compounds are simply constructed differently in nature. The will have very different physical and chemical properties because of the difference in structure.

Examples If you look carefully, you can see that the two compounds have the same numbers of atoms of each element, but the construction is obviously different. We will need to develop a method of naming to communicate to the reader how the molecule is built.

Naming These Structural Isomers There are several things that you have to keep in mind. – You need to determine the fundamental “backbone” of the molecule – what is the longest continuous carbon chain? Next – what type of modification is attached – ask yourself what has replaced one of the hydrogen atoms. Then determine the location of the attachment – on which number carbon is the modification attached?

Example This is simply “pentane”. It has 5 carbon atoms in a continuous chain and nothing but hydrogens attached. This is an isomer of pentane. If you look carefully, the longest continuous chain has only 4 carbon atoms. But, there is a CH 3 (a “methyl” group) replacing one of the H atoms on the 2 nd carbon atom in the chain. So, this compound is “2-methyl butane”.

You can have more than 1 attachment: These are actually the same – just different representations of the same molecule – name it …

Going the other way… Draw the following molecules: 3-methyl – 4 ethyl heptane 2,2 – dimethyl propane 3,5 – dimethyl – 6-ethyl decane Now draw them using skeleton notation.

Substituted Alkanes As stated earlier, it is possible to have organic compounds that contain other elements. As in the structural isomers, these “substitutions” will simply replace one or more of the H atoms attached to carbons. When this happens, we actually have a totally new class of organic compound. The names of these new classes of compounds will reflect what has been used to replace an H atom.

Classes of Substituted Alkanes If a halogen atom (F, Cl, Br, I) is used to replace a Hydrogen, the compound is now an “alkyl halide”. Replacing an H with an “OH” (not a hydroxide in this context) will give you an “alcohol” If an H atom is replaced with an “NH 2 ” group, the compound is now an “amine” (related to ammonia). If an H atom is replaced with a “CN” you will have a “nitrile” (note that the carbon chain will have become longer).

Alkyl Halides This is a displayed formula – it shows the 3-dimensional structure – the tetrahedral geometry. More important here is that one of the H atoms has been replaced by a Cl atom. There are two ways to name this: It can be “methyl chloride” It can be “chloromethane”

Consider this one: What are the two ways to name this?

Alcohols Notice that there is an “OH” group attached to a two-carbon chain. The name will have to account for both of those factors. As with the halides, there are two different ways to name this molecule. it can be “ethyl alcohol” it can be simply “ethanol”

Variations within the Substituted Alkanes All three of these have the same chemical formula, but they have different physical and chemical properties because of the location of the OH group. Your naming will have to account for the location.

Amines As stated earlier, replacing an H atoms with an “NH 2 ” group will result in an “amine”. Obviously, the name will have to reflect this change in chemical identity. This one is “ethylamine”.

And finally, Nitriles. You end up with a nitrile when an H atom is replaced with the “CN” group. There are several things to know about these compounds. 1.The “CN” group contains a triple bond – Cambridge expects you to know this. 2.Adding the CN group will typically increase the length of the carbon chain – that will require you to change the name to correspond to the greater number of carbon atoms.

Consider these two compounds: propane Now replace a H atom with the CN group The CN has been added to the end of the propane. The molecule is now named “butanenitrile”