Carbon, bonding tetrahedral

The difference in behavior between aldehydes/ketones and carboxylic acic derivatives is a consequence of structure. Carboxylic acid derivatives have ai acyl carbon bonded to a group -Y that can leave as a stable anion. As soon a the tetrahedral intermediate is formed, the leaving group is expelled to general- a new carbonyl compound. Aldehydes and ketones have no such leaving grouj however, and therefore don t undergo substitution. 

A ferrocenyl-based aldimine chelated dimethylzinc with its imino and oxygen atom to produce the tetrahedral dimethylzinc complex 44. Both zinc-carbon bonds are equidistant (1.974(2) A) and the zinc-oxygen donor bond (2.381(2) A) is substantially longer than the zinc-nitrogen counterpart (2.213(2) A) (Scheme 37).91... 

The zinc atom has almost ideal tetrahedral coordination geometry, with bond angles ranging from 115.37(10)° to 123.60(9)°. The zinc-carbon bond (1.964(3) A) has a similar length as in the neutral [MeZn(OBut)]4 tetramer,106 in which zinc is also tetrahedrally surrounded by one alkyl group and three oxygen atoms. 

When the potassium derivative was allowed to stand in ammonia, single crystals of the diammoniate K2[Zn(C2Fl)4]-2NFl3 63 separated.120 A low-temperature, single crystal X-ray analysis of 63 showed it to consist of tetrahedral [Zn(C2H)4]2 ions, which feature almost linear Zn-C-C units. The zinc-carbon bonds (2.053(3) A) are slightly longer than those in 64, while the carbon-carbon bonds are identical (1.203(4) A) in length. 

Diboracyclopropane 1A may serve as an example to illustrate the principles discussed above. With the carbon atom tetrahedrally coordinated by two hydrogen and two boron atoms its classical structure is well described by the Lewis formula in Scheme 3.2-1. Hyperconjugation between the CH bonds and the formally empty p orbitals at the boron atoms leads to only a relatively minor reorganization of electron density compared with that suggested by the Lewis formula. 

Figure 8.2. (a) Carbon symmetry—tetrahedral isp ) C —C bond length 15.4 nm. (ft) Carbon symmetry trigonal planar (spi) C C bond length- 14.2 nm interplanar... 

Breslow and co-workers elucidated the currently accepted mechanism of the benzoin reaction in 1958 using thiamin 8. The mechanism is closely related to Lapworth s mechanism for cyanide anion catalyzed benzoin reaction (Scheme 2) [28, 29], The carbene, formed in situ by deprotonation of the corresponding thiazolium salt, undergoes nucleophilic addition to the aldehyde. A subsequent proton transfer generates a nucleophilic acyl anion equivalent known as the Breslow intermediate IX. Subsequent attack of the acyl anion equivalent into another molecule of aldehyde generates a new carbon - carbon bond XI. A proton transfer forms tetrahedral intermediate XII, allowing for collapse to produce the a-hydroxy ketone accompanied by liberation of the active catalyst. As with the cyanide catalyzed benzoin reaction, the thiazolylidene catalyzed benzoin reaction is reversible [30]. 

The bond angles of the covalently bonded tetrahedral carbon atoms in both the model hydrocarbon compounds and the corresponding polymers are 109 28, and the lengths of the C—H and C—C bonds in hdpe are 0.109 and 0.154 nm, respectively. The C—H and C—C bond energies are about 98 and 80 keal/mol, respectively. 

---