Conformations

There is said to be "free rotation" about carbon-carbon single bonds.  This is because, as shown in the graphic, the electron density for a single bond is around the bond axis, and rotation of the two bonded atoms and their groups about this axis does not affect the extent of overlap of the bonding orbitals on each of the atoms.

While there are hundreds of possible arrangements of these groups on the two bonded atoms with respect to one another, chemists generally focus on the two extreme cases where the atoms/groups on the bonded carbons are either as close to one another as possible or as far apart as possible.

Eclipsed conformations have the groups bonded to adjacent atoms directly opposite one another. 

Three eclipsed conformations for butane (CH₃CH₂CH₂CH₃) are shown below using a  "sawhorse" representation.

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	rotate front carbon by 120°
eclipsed		eclipsed

rotate front carbon by 120°
	eclipsed

 

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Staggered conformations have the groups bonded to adjacent atoms rotated 60° with respect to one another.
An eclipsed conformation can be converted to a staggered conformation or vice versa by rotating the groups on one carbon by 60° as shown in the first stage below.  Further rotations by 120° give other staggered conformations similar to above where rotations of 120° converted one eclipsed conformation to another.

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	rotate front carbon and its groups by 60° clockwise
eclipsed		staggered gauche (syn adjacent)

rotate front carbon and its groups by 120° clockwise

	rotate front carbon and its groups by 120° clockwise
staggered anti		staggered gauche (adjacent)

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As shown in the diagram just above, the staggered conformations are classed as gauche (syn adjacent) or anti depending on whether the large groups attached to the two carbons are adjacent to one another or opposite one another.