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Tools of the trade: 3-dimensional structures

Organic compounds are 3‐dimensional and their shape depends on the preferred spatial arrangement of the covalent bonds. The shape of organic compounds are best viewed with the aid of molecular models.

To represent the 3‐dimensional shape on paper, we use hashed and wedged lines. A hashed line points behind the plane of the paper, the wedged line points in front, as shown here for methane (CH4).

Methane has a tetrahedral shape, with carbon in the centre and the four hydrogen atoms pointing to the corners of a triangularly‐based pyramid. All the bond angles are 109.5°. (In ethane, H3C–CH3, the bond angles are near, although not exactly at, the tetrahedral value of 109.5°.)

tetrahedral

The structure gets more complicated for larger molecules. For butanal (CH3CH2CH2CHO), like for methane, we see that three carbon atoms have a tetrahedral shape, but the one in the aldehyde functional group (the purple carbon) is different. It has a trigonal planar, or flat, shape – with the aldehyde carbon in the centre, and oxygen, hydrogen and another carbon pointing to the corners of an equilateral triangle. The bond angles are 120°.

trigonalplanar


Hashed versus broken lines

A hashed line, IIIII, indicates that the bond is extending behind the plane of the drawing surface, whereas a broken line (- - - -) indicates that the bond is not a full bond, it is only a partial bond as in a hydrogen bond or a partially formed or broken bond in a transition state (although, sometimes hashed lines are also used for hydrogen bonds).

Typically, you will see hashed lines drawn using small lines changing to gradually larger width lines as you move away from the carbon (to the atom or substituent it is bonded to). However, some chemists dislike this and do the reverse; they argue that because the bond is pointing away from you, the larger lines should be closer to the carbon (and the smaller width lines should be closer to the atom or substituent), as the atom or substituent is further away from you. To avoid any confusion, many stick with equal width hashed lines and find these are easier to draw (clearly) with pen and paper!

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This article is from the free online course:

Exploring Everyday Chemistry

University of York