Polarized multiple bonds lone pairs

On the other hand these are compounds with marked 1,4-dipolar character, having electron lone pairs at the terminal heteroatom and an electrophilic center at C-4 Consequently, they can react with polarized multiple bond systems, even when these are extremely electron-poor [225]... 

Nucleophilic Addition to a Polarized Multiple Bond Beta Elimination from an Anion or Lone Pair Rearrangement of a Carbocation Rearrangement with Loss of Leaving Group... 

This reaction is a minor variant of the Ac1e2 in which the electrophile, usually a proton, attacks the lone pair rather than the less available heteroatom-carbon pi bond. The lone pair is protonated, path p.t., to give a highly polarized multiple bond that can be viewed as a stabilized carbocation. The nucleophilic attack on this carbocation could also be viewed as path An, trapping of a cation by a nucleophile, instead of path AdN-... 

This pathway (Fig. 7.15) is very similar to path AdN, with the difference that the nucleophile is poorer and is hydrogen bonded to a base when this pair collides with the polarized multiple bond. In this pathway the electron flow comes from the base to break the Nu-H bond, which in turn enhances the nucleophilicity of the nucleophile s lone pair. This lone pair attacks the multiply bonded carbon, breaks the pi bond, and produces a stable anion similar to path Adf. ... 

Occasionally an arrow in the middle of the flow is forgotten, quite often the breaking of a C-H or 0-H bond. In the left example, the origin of the middle arrow is unclear it appears to come from a lone pair position, but the lone pairs have been omitted. The correct path is the general base-catalyzed addition to a polarized multiple bond. 

Simple pi bonds are usually not good enough nucleophiles to react with polarized multiple bonds. If a Br0nsted acid or a Lewis acid is added to improve the electron sink, then addition can occur via the lone-pair-stabilized carbocation as the sink. Figure 8.7 shows a mechanistic example from a short synthesis of the human hormone estrone. 

The most common representatives of the L-C=Y class of electron sinks are the carboxyl derivatives with Y equal to oxygen. In basic media there is only one pathway the addition-elimination path, path Ad y + Ep (see Section 4.5.1). The leaving group should be a more stable anion than the nucleophile, or the reaction will reverse at the tetrahedral intermediate. A follow-up reaction of a second addition to the polarized multiple bond occasionally occurs. With lone pair sources a second addition is rare because the nucleophile is usually a relatively stable species the second tetrahedral intermediate tends to kick it back out (see Section 9.2). 

Generic process An addition and an elimination have occurred. Medium Definitely basic, predominant anion is hydroxide, plsTabH 15.7, whose pA"a would give a useful proton transfer A"eq up to about p Ta 26. Sources The carbonyl lone pair, water lone pair, and hydroxide anion. Best source Hydroxide anion, a lone pair source can behave as a nucleophile or as a base. Sinks Polarized multiple bond, the aldehyde carbonyl. Acidic Hs Water and the CH2 next to the aldehyde, pA a 16.7, are within range of hydroxide. Leaving groups None. Resonance forms ... 

The most important point to remember in classifying into sources and sinks is to stay flexible. Almost all electron sinks have lone pairs that can serve as sources. Lone pairs on sinks can complex with Lewis acids or be protonated. A polarized multiple bond can serve as an electron-withdrawing group, making protons on adjacent atoms acidic. Looking at both reactive partners can help you decide. If one partner is clearly a sink, like sulfuric acid, then the other partner must serve as a source, even though it may be a polarized multiple bond sink like a carbonyl protonate the carbonyl lone pair as a source. 

In the Lewis acid-base definition, an acid is any species that accepts a lone pair to form a new bond in an adduct. Thus, there are many more Lewis acids than other types. Lewis adds include molecules with electron-deficient atoms, molecules with polar multiple bonds, and metal cations. 

Lewis Acids with Polar Multiple Bonds Molecules that contain a polar double bond also function as Lewis acids. As the electron pair on the Lewis base approaches the partially positive end of the double bond, one of the bonds breaks to form the new bond in the adduct For example, consider the reaction that occurs when SO2 dissolves in water. The electronegative O atoms in SO2 withdraw electron density from the central S, so it is partially positive. The O atom of water donates a lone pair to the S, breaking one of the ir bonds and forming an S—O bond, and a proton is transferred from water to that O. The resulting adduct is sulfurous acid, and the overall process is... 

An atom with a lone pair may add to the atom of a polarized multiple bond. As in the type 3 mechanism, the lone electron pair entering from the left pushes a second electron pair out to the right. This second pair is one of the two bonding electron pairs between A and B, so the original A=B double bond is reduced to an A-B single bond. 

That way, the Distributed Electrostatic Moments based on the ELF Partition (DE-MEP) allows computing of local moments located at non-atomic centres such as lone pairs, a bonds and n systems. Local dipole contributions have been shown to be useful to rationalize inductive polarization effects and typical hydrogen bond interactions. Moreover, bond quadrupole polarization moments being related to a n character enable to discuss bond multiplicities, and to sort families of molecules according to their bond order. 

Most of the (W,W)-carbenes are predicted to be linear and this substitution pattern results in a polarized two-electron three-center n system. Here also, the C—W bonds have some multiple bond character these (W,W)-carbenes are best described by the superposition of two ylidic structures featuring a positive charge at the carbene carbon atom. The most studied carbenes of this type are the transient dicarbomethoxycarbenes and the masked diborylcarbenes. Since no carbenes of the latter type have yet been isolated, they are not included in this chapter. Lastly, the quasilinear (D,W)-carbenes combine both types of electronic interaction. The D substituent lone pair interacts with the py orbital, while the W substituent vacant orbital interacts with the px orbital. These two interactions result in a polarized allene-type system with DC and CW multiple bonds. Good examples of this type of carbene are given by the transient halogenocarboethoxycarbenes and by the stable (phosphino)(silyl)- and (phosphino)(phosphonio)carbenes (see below). 

A review of lone pair effects involving multiple bonds between heavier main group elements contains much of relevance to pj -bonded phosphorus systems. The diphosphene (295) has been shown to undergo cycloaddition reactions with isocyanides, to give the iminodiphosphiranes (296)." A thirtyfive-fold excess of methyl triflate is needed to convert the diphosphene (297) to the salt (298), which is unstable in non-polar solvents. Experimental data show that the P=P bond becomes stronger on alkylation as is the case for N=N compounds. 

Polarity is a molecular property. For polyatomic species, the net molecular dipole moment depends upon the magnitudes and relative directions of all the bond dipole moments in the molecule. In addition, lone pairs of electrons may contribute significantly to the overall value of ji. We consider three examples below, using the Pauling electronegativity values of the atoms involved to give an indication of individual bond polarities. This practice is useful but must be treated with caution as it can lead to spurious results, e.g. when the bond multiplicity is not taken into account when assigning a value of x - Experimental values of molecular electric dipole moments are determined by microwave spectroscopy or other spectroscopic methods. 

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