Examples of ortho in the following topics:
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- Snieckus (University of Waterloo, Ontario, Canada) the ortho-lithiation of functionally substituted aromatic rings has proven to be a powerful technique for regioselective synthesis.
- The following equation illustrates this Directed ortho Metalation (DoM) reaction, where DMG refers to a directing metalation group and E+ is an electrophile.
- Electrophilic substitution of aromatic rings generally gives a mixture of ortho and para substitution products when an existing substituent activates the ring or meta products when the substituent is deactivating.
- The second diagram above shows the iodination via directed ortho metalation of anisole will be shown.
- The ortho isomer is the sole product provided excess iodine is avoided.
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- In both cases the charge distribution in the benzene ring is greatest at sites ortho and para to the substituent.
- Toluene gives 58.5% ortho-nitrotoluene, 37% para-nitrotoluene and only 4.5% of the meta isomer.
- The increased bulk of the tert-butyl group hinders attack at the ortho-sites, the overall product mixture being 16% ortho, 8% meta and 75% para-nitro product.
- Although chlorobenzene is much less reactive than benzene, the rate of ortho and para-substitution greatly exceeds that of meta-substitution, giving a product mixture of 30% ortho and 70% para-nitrochlorobenzene.
- Consequently, all these substituents direct substitution to ortho and para sites.
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- This phenolic acidity is further enhanced by electron-withdrawing substituents ortho and para to the hydroxyl group, as displayed in the following diagram.
- Furthermore additional nitro groups have an additive influence if they are positioned in ortho or para locations.
- As noted in our earlier treatment of electrophilic aromatic substitution reactions, an oxygen substituent enhances the reactivity of the ring and favors electrophile attack at ortho and para sites.
- Supporting evidence that the phenolate negative charge is delocalized on the ortho and para carbons of the benzene ring comes from the influence of electron-withdrawing substituents at those sites.
- The additional resonance stabilization provided by ortho and para nitro substituents will be displayed in the third diagram above.
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- This is illustrated in the first diagram below for a β-hydroxyl substituent, examples on the left, and an ortho-tolyl ketone on the right.
- Careful studies of the photoenolization of ortho-alkyl benzophenones and acetophenones have enhanced our understanding of this deceptively simple transformation.
- The second diagram shows the behavior of ortho-methylacetophenone.
- The former has a much longer lifetime (τ) than the latter since it must undergo a conformational change before γ-H abstraction from the ortho-methyl substituent can take place.
- Unlike benzophenone itself, the ortho substituted compound in the upper left corner does not undergo any pinacol reduction.
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- If each of the phenyl rings of a biphenyl has two different ortho or meta substituents (one may be hydrogen), even the twisted 90º dihedral angle conformer becomes chiral.
- The ease with which this interconversion occurs will depend on the size of the ortho substituents, since these groups must slide past each other.
- Although these biphenyls have identical ortho substituents, the meta nitro substituent adjacent to the methoxyl group in C exerts a buttressing influence that increases the effective size of that ortho substituent.
- The right hand compound is heavily ortho-substituted and most certainly resists assuming a planar configuration.
- However, the right benzene ring has two identical ortho substituents, so the stable 90º dihedral angle conformer has a plane of symmetry.
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- The following equation illustrates how this characteristic of the sulfonic acids may be used to prepare the 3-bromo derivative of ortho-xylene.
- The strongest activating and ortho/para-directing substituents are the amino (-NH2) and hydroxyl (-OH) groups.
- Although the activating influence of the amino group has been reduced by this procedure, the acetyl derivative remains an ortho/para-directing and activating substituent.
- However, the overall influence of the modified substituent is still activating and ortho/para-directing.
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- It states that an electron-donating substituent generally accelerates substitution and directs reactivity toward the positions that are ortho and para to it on the ring, while an electron-withdrawing substituent will slow reaction progress and favor the meta position on the ring.
- EAS occurs ortho or para to electron donating groups, such as amines, due to the stabilization of the intermediate positive charge.
- As an exercise, draw out the stabilization of the positive charge when ortho substitution occurs.
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- Note that if two different sites are favored, substitution will usually occur at the one that is least hindered by ortho groups.
- In examples 4 through 6, oppositely directing groups have an ortho or para-relationship.
- Example 6 is interesting in that it demonstrates the conversion of an activating ortho/para-directing group into a deactivating meta-directing "onium" cation [–NH(CH3)2(+)] in a strong acid environment.
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- In the case of disubstituted benzenes, the prefixes ortho, meta & para are commonly used to indicate a 1,2- or 1,3- or 1,4- relationship respectively.
- Some disubstituted toluenes have singular names (e.g. xylene, cresol & toluidine) and their isomers are normally designated by the ortho, meta or para prefix.
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- The presence of electron-withdrawing groups (such as nitro) ortho and para to the chlorine substantially enhance the rate of substitution, as shown in the set of equations presented on the left below.
- The sites over which the negative charge is delocalized are colored blue, and the ability of nitro, and other electron withdrawing, groups to stabilize adjacent negative charge accounts for their rate enhancing influence at the ortho and para locations.