Unshared pairs

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)... 

It will always be true that an oxygen with two covalent bonds and two unshared pairs has no formal charge... 

The green oxygen m Figure 1 5 owns three unshared pairs (six electrons) and shares two electrons with nitrogen to give it an electron count of seven This is one more than the number of electrons m the valence shell of an oxygen atom and so its formal charge is —1... 

FIGURE 1 5 Counting electrons in nitnc acid The electron count of each atom is equal to half the number of electrons it shares in covalent bonds plus the number of electrons in its own unshared pairs... 

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... 

As you practice you will begin to remember patterns of electron distribution A neutral oxygen with two bonds has two unshared electron pairs A neutral nitro gen with three bonds has one unshared pair ... 

The tetrahedral geometry of methane is often explained with the valence shell electron pair repulsion (VSEPR) model The VSEPR model rests on the idea that an electron pair either a bonded pair or an unshared pair associated with a particular atom will be as far away from the atom s other electron pairs as possible Thus a tetrahedral geomehy permits the four bonds of methane to be maximally separated and is charac terized by H—C—H angles of 109 5° a value referred to as the tetrahedral angle... 

The H—O—H angle m water (105°) and the H—N—H angles m ammonia (107°) are slightly smaller than the tetrahedral angle These bond angle contractions are easily accommodated by VSEPR by reasoning that electron pairs m bonds take up less space than an unshared pair The electron pair m a covalent bond feels the attractive force of... 

Water (H2O) 105 H V. 0— / Oxygen has two bonded pairs + two unshared pairs Tetrahedral Bent <4... 

Ammonia (NH3) 107 H / Nitrogen has three bonded pairs + one unshared pair Tetrahedral Trigonal pyramidal ... 

Bonded pair bonded pair > Unshared pair bonded pair > Unshared pair unshared pair Least repulsive Most repulsive... 

In the equation shown the base uses an unshared pair of electrons to remove a proton from an acid The base is converted to its conjugate acid, and the acid is converted to... 

Now use curved arrows to track electron flow Move electrons from sites of high electron density (unshared pairs negative charge) toward sites of lower electron density... 

One of the unshared pairs of the hydroxide oxygen is used to form a covalent bond to the positively polar ized proton of hydrogen fluoride The covalent bond betwen H and F in hydrogen fluoride breaks with the pair of electrons in this bond becoming an unshared pair of fluoride ion... 

An unshared pair of electrons from the Lewis base is used to form a covalent bond between the Lewis acid and the Lewis base The Lewis acid and the Lewis base are shown as ions m the equation but they need not be If both are neutral molecules the analogous equation becomes... 

The carbon atom m bromomethane can accept an electron pair if its covalent bond with bromine breaks with both electrons m that bond becoming an unshared pair of bromide ion Thus bromomethane acts as a Lewis acid m this reaction... 

Recall that the carbon atom of carbon dioxide bears a partial positive charge because of the electron attracting power of its attached oxygens When hydroxide ion (the Lewis base) bonds to this positively polarized carbon a pair of electrons in the carbon-oxygen double bond leaves carbon to become an unshared pair of oxygen... 

Lewis bases use an unshared pair to form a bond to some other atom and are also referred to as nucleophiles ( nucleus seekers ) Conversely Lewis acids are elec trophiles ( electron seekers ) We will use these terms hundreds of times throughout the remaining chapters... 

Describe the bonding in ammonia assuming sp hybridization of nitrogen In what kind of orbital is the unshared pair What orbital overlaps are involved in the N—H bonds , ... 

In the corresponding ionizations of ethylene and ethane the unshared pair occupies an orbital with 33% sp ) and 25% sp ) s character respectively Terminal alkynes (RC=CH) resemble acetylene m acidity... 

In pyrrole on the other hand the unshared pair belonging to nitrogen must be added to the four tt electrons of the two double bonds m order to meet the six tt elec tron requirement As shown m Figure 11 166 the nitrogen of pyrrole is sp hybridized and the pair of electrons occupies a p orbital where both electrons can participate m the aromatic tt system... 

Pyridine and pyrrole are both weak bases but pyridine is much more basic than pyrrole When pyridine is protonated its unshared pair is used to bond to a proton and... 

FIGURE 1116 (a) Pyridine has six tt electrons plus an unshared pair in a nitrogen sp orbital (b) Pyrrole has six tt electrons (c) Furan has six tt electrons plus an unshared pair in an oxygen sp orbital which is perpendicular to the tt system and does not interact with it... 

The oxygen m furan has two unshared electron pairs (Figure 11 16c) One pair is like the pair m pyrrole occupying a p orbital and contributing two electrons to complete the SIX TT electron requirement for aromatic stabilization The other electron pair m furan IS an extra pair not needed to satisfy the 4n + 2 rule for aromaticity and occupies an sp hybridized orbital like the unshared pair m pyridine The bonding m thiophene is similar to that of furan... 

Unlike the case of benzene in which ionization involves loss of a tt electron from the ring electron impact induced ionization of chlorobenzene involves loss of an elec tron from an unshared pair of chlorine The molecular ion then fragments by carbon-chlorine bond cleavage... 

Thus with dihalocarbenes we have the interesting case of a species that resem bles both a carbanion (unshared pair of electrons on carbon) and a carbocation (empty p orbital) Which structural feature controls its reactivity s Does its empty p orbital cause It to react as an electrophile s Does its unshared pair make it nucleophilic s By compar mg the rate of reaction of CBi2 toward a series of alkenes with that of typical electrophiles toward the same alkenes (Table 14 4) we see that the reactivity of CBi2... 

FIGURE 14 3 (a) The unshared electron pair occupies an sp hybridized orbital in dichlorocarbene There are no electrons in the unhybridized p orbital (b) An electrostatic potential map of dichlorocarbene shows negative charge is concentrated in the region of the unshared pair and positive charge above and below the carbon... 

Write a resonance form for guanine in which the six membered ring has an electronic structure analogous to benzene Show all unshared pairs... 

The sp hybrid state of nitrogen is just like that of carbon except nitrogen has one more electron Each N—H bond in NH3 involves overlap of an sp hybrid orbital of N with a li orbital of hydrogen The unshared pair of NH3 occupies an sp orbital... 

The unshared pairs of electrons on hydroxyl oxygens seek electron deficient centers. Alkylphenols tend to be less nucleophiUc than aUphatic alcohols as a direct result of the attraction of the electron density by the aromatic nucleus. The reactivity of the hydroxyl group can be enhanced in spite of the attraction of the ring current by use of a basic catalyst which removes the acidic proton from the hydroxyl group leaving the more nucleophiUc alkylphenoxide. 

The unshared pair of electrons on the nitrogen atom provides the basic character to the fatty amines. Basicity of amines has been deterrnined as... 

The protonated azirine system has also been utilized for the synthesis of heterocyclic compounds (67JA44S6). Thus, treatment of (199) with anhydrous perchloric acid and acetone or acetonitrile gave the oxazolinium perchlorate (207) and the imidazolinium perchlorate (209), respectively. The mechanism of these reactions involves 1,3-bond cleavage of the protonated azirine and reaction with the carbonyl group (or nitrile) to produce a resonance-stabilized carbonium-oxonium ion (or carbonium-nitrilium ion), followed by attack of the nitrogen unshared pair jf electrons to complete the cyclization. 

Most heterocyclic anions may be considered to be derived by loss of a proton from a parent compound, which is therefore the conjugate acid. Such anions have at least one unshared pair of electrons at the anionic site. They are named by appending the suffix -ide , with elision of a terminal e (lUPAC recommendation RC-83.1.1), as in (190)-(193). The site may be specified by a locant placed immediately before the suffix, and so chosen as to be as low as possible consistent with the numbering of the skeleton of the parent compound. The locant may be omitted in order to designate an equilibrating mixture of positionally isomeric anions, which is what one usually obtains in practice. The anion of piperidine is often informally referred to as piperidide . 

Discuss these trends in relation to the interaction of the nitrogen unshared pair and the C=N electrons with the substituent. 

Another significant structural effect that inrparts high nucleophilicity is the alpha effect. It is observed that atoms which are directly bonded to an atom with one or rhore unshared pairs of electrons tend to be stronger nucleophiles than would othermse be expected. Examples in Table 5.7 include H02, which is more nucleophilic than HO , and... 

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