Polar arrow

The basic reaction remains the same when substituents are present, as illustrated in Figure 4-16. In this example, the aldehyde is an electron-withdrawing group (the electronegative oxygen pulls electron density away from the double bond). The polarity arrow illustrates this electron shift. This shift of electron density speeds up the reaction (a lower temperature is necessary). 

Plan (a) We use Figure 9.20 to find the EN values of the bonded atoms and point the polar arrow toward the negative end. (b) Each choice has H bonded to an atom from Period 2. EN increases across a period, so the polarity is greatest for the bond whose Period 2 atom is farthest to the right. 

Rank the members of each set of compounds in order of increasing ionic character of their bonds. Use a polar arrow to indicate the bond polarity of each ... 

Recombination Cancellation and F-pair Polarization. To discuss more quantitatively the NMR time dependence arising from these processes the reaction scheme in Figure 5 is considered in which the polarization arrows have been omitted for convenience. 

Fig. 7.10. (a) tapping mode image of a polyimide layer patterned in crossed directions. (b) micrograph of a patterned twisted nematic pixel under crossed polarizers. Arrows indicate the scan directions and the number shows the scan order. 

Figure 4.1 Electron distribution in moiecules of H2 and H2O. A, In H2, the identical nuclei attract the electrons equally. The central region of higher electron density (red) is balanced by the two outer regions of lower electron density (blue). B, In H2O, the O nucleus attracts the shared electrons more strongly than the H nucleus. C, In this ball-and-stick model, a polar arrow points to the negative end of each O—H bond. D, The two polar O—H bonds and the bent shape give rise to the polar H2O molecule.

Bent molecular shape. The sequence of the H—O—H atoms in water is not linear the water molecule is bent with a bond angle of 104.5°. In the baU-and-stick model of Figure 4.1C, the polar arrow points to the negative pole, and the tail, shaped like a plus sign, marks the positive pole. 

Problem (a) Use a polar arrow to indicate the polarity of each bond N—H, F—N, I—Cl. (b) Rank the following bonds in orda- of increasing polarity and decreasing percent ionic character H—N, H—O, H—C. 

Plan We draw and name the molecular shape and point a polar arrow toward the atom with higher EN in each bond. If the bond polarities balance one another, the molecule is nonpolar if they reinforce each other, we show the direction of the molecular polarity. 

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