So again in Chapter 4 David Klein book, we have this organic molecule with a t-butyl group (just as an aside, this is t-butyl ethane, right?

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Now how is it that the correct Newman projection must be the following? enter image description here

What I glean from the Lewis structure is that because there are no wedges or dashed lines, the methyl groups ought to be in the same plane. How can there be a methyl group jutting out upwards and to the right, and another methyl group jutting out upwards and to the left in the t-butyl group and not be illustrated as such with dashed lines and wedges? Thanks in advance!

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    $\begingroup$ It's not " t-butyl ethane", it's 2,2‐dimethylbutane. Cram bonds are not always drawn, but it doesn't mean that all the atoms belong to the same plane, you should also apply common sense sometimes. Check out the 3D structure to get better understanding of how atoms are arranged. $\endgroup$ – andselisk Dec 29 '18 at 3:29
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    $\begingroup$ @andselisk thanks for the corrections! Still trying to wrap my head around proper nomenclature as you can see. Oh gotcha, didn't know about that, I will be sure to keep that in mind going forward. Also, awesome, I didn't know about that webpage, thank you so much! I will be sure to use in it in the future for more Newman projection conundrums I have. $\endgroup$ – Environmental Enthusiast Dec 29 '18 at 3:43
  • $\begingroup$ The three front methyl groups identify a plane as always with tree points. This is pedantic but related as for this is exactly how Fisher proj. shall be read or sketched. There will never be any cram bond as for all are tilt pointing to the observer. The presence of a double bond will make the reading obvious as well. $\endgroup$ – Alchimista Dec 29 '18 at 14:36

Lewis structures and skeletal structures show which atoms are connected to which other atoms, and the type of bonds (single, double, triple). They do not show the shape (geometry) of the molecule.

For example, the two skeletal structures below refer to the same molecule, butane:

propane in two conformations

When there is ambiguity (i.e. multiple molecules with the same skeletal structure), you can add information about the stereochemistry by adding dashes bonds and wedges. In your case, the molecule is achiral, so that does not apply.

To figure out the geometry, remember that carbon atoms with four single bonds have a tetrahedral geometry. You get other geometries when there is a double bond (trigonal planar) or a triple bond (linear) or other, more unusual features associated with a carbon atom (lone pair, non-zero formal charge, radicals, two double bonds).


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