Octets

In the example in Figure 2-19, the oxygen atom 3 has 2- 4 (row) + 2 + 4 (column) - 4 (diagonal element) = 8 electrons. This shows that the ox> gen atom obeys the octet rule. 

Figure 2-19. The BE-matriK of ethanal allows one to determine tine number of valence electrons (the sum of each row) on the atoms and to validate the octet rule,...

Protonated methane (CH ) does not violate the octet rule of carbon. A bonding electron pair (responsible for covalent bonding between C and H atoms) is forced into sharing with the proton, resulting in 2 electron-3 center bonding (2e-3c) (see Chapter 10). Higher alkanes are protonated similarly. 

In some situations the nitro group behaves as if it exerted its influence mainly by the inductive process, but in nitration its behaviour seems to place it with this group of —I —M substituents. The precise way in which a sulphone group is described depends on how much weight is given to the ability of the sulphur atom to expand its octet the positive... 

Lewis s concept of shared electron parr bonds allows for four electron double bonds and SIX electron triple bonds Carbon dioxide (CO2) has two carbon-oxygen double bonds and the octet rule is satisfied for both carbon and oxygen Similarly the most stable Lewis structure for hydrogen cyanide (HCN) has a carbon-nitrogen triple bond... 

Multiple bonds are very common m organic chemistry Ethylene (C2H4) contains a carbon-carbon double bond m its most stable Lewis structure and each carbon has a completed octet The most stable Lewis structure for acetylene (C2H2) contains a carbon-carbon triple bond Here again the octet rule is satisfied... 

It will always be true that a nitrogen with four covalent bonds has a formal charge of + 1 (A nitrogen with four co valent bonds cannot have unshared pairs because of the octet rule)... 

Add electrons in pairs so that as many atoms as possible have eight electrons (Hydrogen is limited to two electrons) When the number of electrons is insufficient to provide an octet for all atoms assign electrons to atoms in order of decreasing electro negativity... 

If one or more atoms have fewer than eight elec trons use unshared pairs on an adjacent atom to form a double (or triple) bond to complete the octet... 

Lewis structures in which second row elements own or share more than eight valence electrons are especially unstable and make no contribution to the true structure (The octet rule may be ex ceeded for elements beyond the second row)... 

When two or more structures satisfy the octet rule the most stable one is the one with the smallest separation of oppositely charged atoms... 

The two Lewis structures D and E of methyl nitrite satisfy the octet rule... 

Among structural formulas in which the octet rule IS satisfied for all atoms and one or more of these atoms bears a formal charge the most stable reso nance form is the one in which negative charge re sides on the most electronegative atom... 

Section 1 3 The most common kind of bonding involving carbon is covalent bond ing A covalent bond is the sharing of a pair of electrons between two atoms Lewis structures are written on the basis of the octet rule, which limits second row elements to no more than eight electrons m their valence shells In most of its compounds carbon has four bonds... 

There is ample evidence from a variety of sources that carbocations are mterme diates m some chemical reactions but they are almost always too unstable to isolate The simplest reason for the instability of carbocations is that the positively charged car bon has only six electrons m its valence shell—the octet rule is not satisfied for the pos itively charged carbon... 

Free radicals are species that contain unpaired electrons The octet rule notwithstand mg not all compounds have all of their electrons paired Oxygen (O2) is the most famil lar example of a compound with unpaired electrons it has two of them Compounds that have an odd number of electrons such as nitrogen dioxide (NO2) must have at least one unpaired electron... 

The chief reason why ethylenebromonium ion m spite of its strained three membered ring IS more stable than 2 bromoethyl cation is that both carbons and bromine have octets of electrons whereas one carbon has only six electrons m the carbocation... 

Of the two resonance forms A and B A has only six electrons around its positively charged carbon B satisfies the octet rule for both carbon and oxygen It is more stable than A and more stable than a carbocation formed by protonation of a typical alkene... 

Most stable resonance form oxygen and carbon have octets of electrons... 

Oxygen stabilized carbocations of this type are far more stable than tertiary carbocations They are best represented by structures m which the positive charge is on oxygen because all the atoms have octets of electrons m such a structure Their stability permits them to be formed rapidly resulting m rates of electrophilic aromatic substitution that are much faster than that of benzene... 

Many transition metal complexes including Ni(CO)4 obey the 18 electron rule, which IS to transition metal complexes as the octet rule is to mam group elements like carbon and oxygen It states that... 

Isomtriles are stable often naturally occumng compounds that contain a divalent carbon An example is axisonitnle 3 which can be isolated from a species of sponge and possesses anti malanal activity Write a resonance form for axisonitnle 3 that satisfies the octet rule Don t for get to include formal charges... 

A particularly stable resonance form satisfies the octet mle for carbon and oxygen... 

Lewis structure (Section 1 3) A chemical formula in which electrons are represented by dots Two dots (or a line) be tween two atoms represent a covalent bond in a Lewis structure Unshared electrons are explicitly shown and sta ble Lewis structures are those in which the octet rule is sat isfied... 

Octet rule (Section 1 3) When forming compounds atoms gain lose or share electrons so that the number of their va lence electrons is the same as that of the nearest noble gas For the elements carbon nitrogen oxygen and the halo gens this number is 8... 

Ylide (Section 17 12) A neutral molecule in which two oppo sitely charged atoms each with an octet of electrons are di rectly bonded to each other The compound... 

The valence theory (4) includes both types of three-center bonds shown as well as normal two-center, B—B and B—H, bonds. For example, one resonance stmcture of pentaborane(9) is given in projection in Figure 6. An octet of electrons about each boron atom is attained only if three-center bonds are used in addition to two-center bonds. In many cases involving boron hydrides the valence stmcture can be deduced. First, the total number of orbitals and valence electrons available for bonding are determined. Next, the B—H and B—H—B bonds are accounted for. Finally, the remaining orbitals and valence electrons are used in framework bonding. Alternative placements of hydrogen atoms require different valence stmctures. 

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