Organic Chemistry Lecture 2 PDF

## Heterocyclic rings fused with carbocyclic systems: - **Heterocyclic rings fused with carbocyclic systems.** You start naming heterocyclic rings that are attached to a benzene ring or a carbon-only ring. The name is divided into two parts: 1. **The prefix**. This is the name of the...

## Heterocyclic rings fused with carbocyclic systems: - **Heterocyclic rings fused with carbocyclic systems.** You start naming heterocyclic rings that are attached to a benzene ring or a carbon-only ring. The name is divided into two parts: 1. **The prefix**. This is the name of the benzene ring (**benzo**). If the benzene ring has branches you need to know how to give it numbers and name the branches. If the ring is saturated it's called **Cyclo** + number of carbon atoms using our naming system (...propyl, butyl, penty, and so on). 2. **The suffix**. This is the name of the heterocyclic ring as we defined in the previous section. The only difference is that if this ring has another common name it can be used instead of the IUPAC name. 3. The bond between the two rings is called **fusion side**. This bond has a letter (...a,b,c,d) which you will use to name it. When you number the heterocyclic ring (we will clarify how to do that soon), write that letter between brackets, between the suffix and the prefix. ## IUPAC Name: - **The IUPAC name of fused ring heterocycles consists of:** - The hetero ring is the suffix. - The carbocyclic ring is the prefix and fused benzene is called benzo. - The position of the ring junction (side of fusion). - The numbering of the complete fused system. - **How do you number the two rings?** - This is to give a number to the bond and if there is a branch on any ring, you write its number before it. - First, you need to determine what letter the fusion side will take. - **The heterocyclic ring is the basis**, so you cover the benzene ring with your hand from the fusion side, and start numbering the heterocyclic ring. - **External numbering (don't number the bond connecting the rings):** start from the first atom next to the fusion site, even if it's not the heteroatom. You move in a direction that gives the heteroatom the lowest possible number (like the example). - What if the heteroatom is right next to the fusion site? Then, the heteroatom gets number 1. This numbering allows you to determine the position of a branch on the heterocyclic ring. - Numbering starts from the atom next to the fusion, giving the hetero atoms the lowest possible numbers. ## Numbering bonds: - The second type of numbering assigns letters to each bond (..., a,b,c,d). It starts from the first bond with the heteroatom (N.O.S) and goes towards the fusion side, but there's a crucial requirement: you must go in a direction that passes through all the heteroatoms first on your way to the fusion site. If there's only one heteroatom, you go on the shortest path towards the fusion site. ## Additional Examples: - **Thiophene:** - The heterocyclic ring has a common name, **thiophene**. - If you use the IUPAC name it would by **thiole**. - Start numbering the ring from the first atom towards the fusion side. - This bond will be "b." After that, you number the ring from S because if there are any branches you need to know where they are. - So the complete name will be **benzo(b)thiophene** or **benzo(b)thiole**. - *5 -methylbenzo(b)thiole/thiophene*: - The example is the same but has a branch and the external numbering will be useful. - You use the same name but add the branch. So, the new full name will be **5 -methylbenzo(b)thiole/thiophene**. - **Oxazine:** - The heterocyclic ring doesn't have a common name so you name it like the previous sections and number it. - *1,3 -oxazine,* but this is just the name. - There is **obligatory saturation** on atom 4 because the O atom only has two bonds. This is considered like a branch on the ring. - Start numbering the ring from N towards O first. - You have to number the bonds so that they pass through all the heteroatoms first - *The fusion side will be “d”*. This way you combine the whole name and get * **4-H benzo(d) 1,3-oxazine**.* ## Heterocyclic rings fused with another heterocyclic system - Now, we will discuss the second part: naming a compound formed by two heterocyclic rings, where each is made of at least one heteroatom (N, O, S). - We will follow the same rules for numbering and naming the compound, but with a small modification or additions. - What's different from naming a heterocyclic ring with a benzene ring? The difference is that I knew the heterocyclic ring was the **parent ring** and its name would be the compound's suffix without any additional requirements for choosing it, besides being a heterocyclic ring. - However, we have two heterocyclic rings here, so we need to determine which ring is the **parent** and which ring is the **suffix.** - **The parent ring is the one which has N**. If the other ring has any number of atoms except N, the parent ring is the N-containing ring. - With no N, we consider the ring which has the highest heteroatom priority using the priority order: O > S > Se > N > P > Si > B > Hg ....etc. - If both contain the same number and type of heteroatoms (like N or other heteroatoms)? Then the ring with more atoms will be the parent. - The ring having the most heteroatoms, or the one with more different heteroatoms, is usually the parent, considering the first rule. - The last rule: if all the above are similar then the ring which contains the most carbon atoms directly linked to the fusion bond is the parent. - **The parent ring name always goes as a suffix**. - We will now discuss numbering to be able to determine the name of the compound completely and correctly. - There are three types of numbering: two internal (bond numbers) and one external (atom numbers). - **External numbering:** it's like before, starts from the first atom next to the fusion bond on the parent ring. You move in a direction that gives a low number to the hetero atoms within that ring. - Note that you don't number the bond connecting the two rings except if that bond contains a hetero atom. If it does then that hetero atom gets a simple number during its turn. - **The first internal numbering is by letters** (...,a,b,c,d). Use this to determine which letter your fusion bond will have. You start numbering from the heteroatom in the parent ring. You need to move in a direction so that you pass through every heteroatom until you reach the fusing bond. - **The second type of numbering is by numbers** and it's only for the second ring. It starts from the closest heteroatom and goes in the shortest direction to reach the fusion bond so that the fusion bond gets two numbers. - So, the name will be comprised of: - parent ring name + [first internal number - second internal number + second ring name + branch(es)] - The parent ring sides are numbered first according to the numbering system (...,a,b,c. starting from (a) for side 1, 2) and verify the letter for the fusion side. - The ring system is numbered and the numbered atoms of the fusion bonds are indicated. The numbers are chosen to be as low as possible and the numbering order corresponds to the direction of the lettering for the parent ring. - **Examples:** - There are two rings, one with N and the other with no N, so N-containing ring is the parent. It has a common name, **Pyrazole**. We don't need to do external numbering because there are no branches. - **The first internal numbering (on the parent ring):** the bond between 2 gets the letter "a" because we start at the heteroatom and go to the nearest heteroatom. Then we choose the closest direction to the fusion bond and it gets the letter "c." - **The second internal numbering:** starts from the heteroatom on the other ring and goes in the shortest direction towards the fusion bond. This will be numbers **2 and 3**. - The full name will be: **pyrano[2,3-c]pyrazole.** - Remember to keep the bond numbers order in the same direction as the first one. So, if the first numbering is from the top the second numbering should also be from the top, even if the numbering starts from the bottom. - The second example, there's no N so we use the highest priority heteroatom. The furan ring is the highest. We then do the internal numbering as shown in the figure (we don't need external numbering because there are no branches), so, the name will be: **thieno [2,3-b] furan.** - In the third example, the heteroatoms are identical and the rings have the same number of carbon atoms. The ring with the most carbon atoms directly linked to the fusion is the parent. - **Pyridazine.** - We need to do the internal numbering and we can do it from the top or the bottom because the bond order doesn’t matter. The bond will take numbers 2 and 3. - So, the full name will be: **Pyrazino[2,3-d]pyridazine**. - We have two rings, both with N. We choose the one that contains more heteroatoms as the parent. -We start numbering from the first atom next to the fusion site, giving the heteroatoms in the parent ring the lowest numbers, and then, we number the second ring. - The full name will be: **6H-2-aminopyrido[2,3-d]pyrimidin-5-one**. - Let's practice this with two examples: - The red structure (with numbers): **6H-2amino pyrido[2,3-d] pyrimidine-5-one** - The green structure (with numbers): **Pyrazino[2,3-d]pyridazine**.