Which skeleton below is a structural isomer of this compound

Conformational isomers , also known as conformers, differ from one another by their rotation around a single bond. Rotations occur freely around single carbon-carbon bonds. Unlike double and triple bonds, which are "locked" in their orientation, single bonds have no such restrictions. A structural isomer , also known as a constitutional isomer, is one in which two or more organic compounds have the same molecular formulas but different structures. The two molecules below have the same chemical formula, but are different molecules because they differ in the location of the methyl group.

Alkenes can also demonstrate structural isomerism. In alkenes, there are multiple structural isomers based on where in the chain the double bond occurs. The condensed structural formulas of 1-butene and 2-butene show this.

The number in the name of the alkene refers to the lowest numbered carbon in the chain that is part of the double bond. Stereoisomers have the same connectivity in their atoms but a different arrangement in three-dimensional space. There are different classifications of stereoisomers depending on how the arrangements differ from one another. Notice that in the structural isomers, there was some difference in the connection of atoms.

For example, 1-butene has a double bond followed by two single bonds while 2-butene has a single bond, then a double bond, then a single bond. A stereoisomer will have the same connectivity among all atoms in the molecule.

With a molecule such as 2-butene, a different type of isomerism called geometric isomerism can be observed. Geometric isomers are isomers in which the order of atom bonding is the same but the arrangement of atoms in space is different.

The double bond in an alkene is not free to rotate because of the nature of the bond. Therefore, there are two different ways to construct the 2-butene molecule see figure below. The image below shows the two geometric isomers, called cis butene and trans butene. The cis isomer has the two single hydrogen atoms on the same side of the molecule, while the trans isomer has them on opposite sides of the molecule.

In both molecules, the bonding order of the atoms is the same. In order for geometric isomers to exist, there must be a rigid structure in the molecule to prevent free rotation around a bond.

This occurs with a double bond or a ring. In addition, the two carbon atoms must each have two different groups attached in order for there to be geometric isomers. Propene see figure below has no geometric isomers because one of the carbon atoms the one on the far left involved in the double bond has two single hydrogens bonded to it. Physical and chemical properties of geometric isomers are generally different.

As with alkenes, alkynes display structural isomerism beginning with 1-butyne and 2-butyne. However, there are no geometric isomers with alkynes because there is only one other group bonded to the carbon atoms that are involved in the triple bond. Stereoisomers that are not geometric isomers are known as optical isomers. Optical isomers differ in the placement of substituted groups around one or more atoms of the molecule.

They were given their name because of their interactions with plane-polarized light. Optical isomers are labeled enantiomers or diastereomers.

Enantiomers are non-superimposable mirror images. If you had a model of a molecule in front of you, you would have to take it to pieces and rebuild it if you wanted to make an isomer of that molecule. If you can make an apparently different molecule just by rotating single bonds, it's not different - it's still the same molecule. In structural isomerism, the atoms are arranged in a completely different order.

This is easier to see with specific examples. What follows looks at some of the ways that structural isomers can arise. The names of the various forms of structural isomerism probably don't matter all that much, but you must be aware of the different possibilities when you come to draw isomers. These isomers arise because of the possibility of branching in carbon chains.

In one of them, the carbon atoms lie in a "straight chain" whereas in the other the chain is branched. Be careful not to draw "false" isomers which are just twisted versions of the original molecule.

For example, this structure is just the straight chain version of butane rotated about the central carbon-carbon bond.

You could easily see this with a model. This is the example we've already used at the top of this page. Pentane, C 5 H 12 , has three chain isomers. If you think you can find any others, they are simply twisted versions of the ones below. If in doubt make some models. In position isomerism, the basic carbon skeleton remains unchanged, but important groups are moved around on that skeleton.

For example, there are two structural isomers with the molecular formula C3H7Br. In one of them the bromine atom is on the end of the chain, whereas in the other it's attached in the middle. If you made a model, there is no way that you could twist one molecule to turn it into the other one. You would have to break the bromine off the end and re-attach it in the middle.

At the same time, you would have to move a hydrogen from the middle to the end.