tetrahedral
Summary
Tetrahedral describes an atom with four groups or electron domains arranged at about 109.5°, as in an sp³ carbon. Most testable: a planar sp² carbonyl carbon becomes tetrahedral when a nucleophile adds, forming the tetrahedral intermediate.
Detail
Meaning: A tetrahedral center has four electron domains directed toward the corners of a tetrahedron, with bond angles of about 109.5°. A carbon with four single bonds is sp³ hybridized and tetrahedral. In VSEPR, lone pairs count as domains, so the electron geometry of NH₃ and H₂O is tetrahedral even though their molecular shapes are trigonal pyramidal and bent.
On the MCAT: A common setup is a trigonal planar, sp² carbonyl carbon (120°) attacked by a nucleophile, which turns it into a tetrahedral sp³ carbon. This is the tetrahedral intermediate of nucleophilic acyl substitution, and the alkoxide formed in addition to aldehydes and ketones. Be ready to predict the change in hybridization and bond angle. Crowding at the new tetrahedral carbon slows attack, so ketones usually react more slowly than aldehydes. A tetrahedral carbon bonded to four different groups is a stereocenter, so chirality questions rely on this geometry.
Common confusion: Students treat the tetrahedral intermediate as a transition state. It is a true intermediate, a species at an energy minimum, which then collapses by expelling a leaving group. Likewise, the SN2 transition state at carbon is trigonal bipyramidal, not tetrahedral.
Sources
- Organic Chemistry (Clayden, Greeves, Warren)
- Organic Chemistry (Klein)
- Organic Chemistry (Wade)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.