Why do sp2 carbons tend to prefer eclipsed conformations whereas sp3 carbons prefer staggered

I am a second year undergraduate and have come across this when studying stereochemistry and conformations and have been unable to find a satisfactory answer. 


I know that $\ce{sp^3}$ carbons prefer anti-periplanar and gauche conformations due to the overlap of filled orbitals with empty orbitals but cannot seem to find an answer for why $\ce{sp^2}$ carbons prefer an eclipsed conformation. I was possibly thinking about the need to avoid $\pi$ electrons however I am not sure. 




I think you are asking about preferred conformations for $\ce{sp^3-sp^3}$ single bonds and $\ce{sp^3-sp^2}$ single bonds. 


You are correct that the staggered (or gauche) conformation is preferred in simple alkanes. This is generally explained by noting that 


* adverse steric interactions are minimized in the staggered conformation, and
* stabilizing [hyperconjugative interactions](http://en.wikipedia.org/wiki/Hyperconjugation) are maximized


![enter image description here](https://i.stack.imgur.com/lqNig.jpg)  

([From: “Isomers of Organic Compounds”, section 24.2 from the book Principles of General Chemistry (v. 1.0).](http://2012books.lardbucket.org/books/principles-of-general-chemistry-v1.0/s28-02-isomers-of-organic-compounds.html))


In the case of the $\ce{sp^3-sp^2}$ single bond as found in propene, toluene, acetaldehyde, etc., the eclipsed conformation is now the more stable.


![enter image description here](https://i.stack.imgur.com/p3fHR.png)  

([Michael Di Maso, Jared Shaw, the UC Davis ChemWiki by University of California](http://chemwiki.ucdavis.edu/Wikitexts/UC_Davis/UCD_Chem_231A%3A_Methods_of_Organic_Synthesis/Content/Conformational_Analysis/Acyclic_Conformations))


Again both steric and electronic arguments can be advanced to explain this preference. The steric argument says let's use the bent model to describe the double bond.


![enter image description here](https://i.stack.imgur.com/IJHl0.gif)


Now replace one of those ethylene hydrogens with a methyl group and note that in the eclipsed conformation (build a model if you can) none of the methyl hydrogens eclipse the hydrogen on the adjacent olefinic carbon or any of the bent bonds. In the staggered conformation all 3 of the methyl hydrogens are eclipsing something, the 2 bent bonds and the $\ce{C-H}$ bond on the adjacent olefinic carbon. The basis of the electronic argument can be found [here](http://chemwiki.ucdavis.edu/Wikitexts/UC_Davis/UCD_Chem_231A%3A_Methods_of_Organic_Synthesis/Content/Conformational_Analysis/Acyclic_Conformations). Personally, for me, the steric argument is easier to understand.


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