Carbon skeleton changes

Has the carbon skeleton changed —Yes. This indicates a carbocation rearrangement. 

In all of the problems in the previous section, the functional group changed its identity or location, but the carbon skeleton always remained the same. In this section, we will focus on examples in which the carbon skeleton changes. In some cases, the number of carbon atoms in the skeleton increases, and in other cases, the number of carbon atoms decreases. 

The mechanism of the reaction la not known with certainty. It is known from studies utilising as tracer that no change in the carbon skeleton occurs during the reaction, and also that unsaturated hydrocarbons can undergo reactions very similar to those of ketones thus both styiene and phenyl-acetylene can react with sulphur and morpholine to produce phenylaceto-thiomorphoUde, hydrolysis of which yields phenylacetic acid ... 

The simplest way to draw an enantiomer is to redraw the carbon skeleton, but invert all stereocenters. In other words, change all dashes into wedges and change all wedges into dashes. Eor example,... 

Both H and R can attack a ketone or aldehyde to give an alcohol. The main difference is the effect on the carbon skeleton. With H, the carbon skeleton does not change at all. But with R, the carbon skeleton gets larger. We are forming a C—C bond. We will soon see that this is very important for synthesis problems. For now, let s focus on how we can make R in the first place. After all, a negative charge on a carbon atom is not very stable (and therefore not trivial to make). 

Despite the apparent confusion above, rearrangements involving carbocations may be usefully divided into those in which an actual change in carbon skeleton itself does, or does not, take place the former are much the more important but the latter will first be briefly referred to. 

With change in carbon skeleton 5.4.2.1 Neopentyl rearrangements... 

The Clemmensen reduction of j8-diketones (1,3-diketones) is rather complicated. The first step in the reaction of 2,4-pentanedione with zinc amalgam is an intramolecular pinacol reduction leading to a cyclopropanediol. Next the cyclopropane ring is opened in the acidic medium, and a rearrangement followed by a reduction gives the final product, a ketone, with a changed carbon skeleton [924, 925]. The ketone is usually accompanied by small amounts of the corresponding hydrocarbon [924] or an a-hydroxy ketone [925]. 

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