Ortho Meta Para H Nmr

[Su Strawderman]

Althoughthe specifics vary depending on the compound, in simple disubstituted arenes, the three isomers tend to have rather similar boiling points. However, the para isomer usually has the highest melting point, and the lowest solubility in a given solvent, of the three isomers.[1]

Because electron donating groups are both ortho and para directors, separation of these isomers is a common problem in synthetic chemistry. Several methods exist in order to separate these isomers:

The prefixes ortho, meta, and para are all derived from Greek, meaning correct, following, and beside, respectively. The relationship to the current meaning is perhaps not obvious. The ortho description was historically used to designate the original compound, and an isomer was often called the meta compound. For instance, the trivial names orthophosphoric acid and trimetaphosphoric acid have nothing to do with aromatics at all. Likewise, the description para was reserved for just closely related compounds. Thus Jns Jakob Berzelius originally called the racemic form of tartaric acid "paratartaric acid" (another obsolete term: racemic acid) in 1830. The use of the prefixes ortho, meta and para to distinguish isomers of disubstituted aromatic rings starts with Wilhelm Krner in 1867, although he applied the ortho prefix to a 1,4-isomer and the meta prefix to a 1,2-isomer.[6][7] It was the German chemist Karl Grbe who, in 1869, first used the prefixes ortho-, meta-, para- to denote specific relative locations of the substituents on a disubstituted aromatic ring (namely naphthalene).[8] In 1870, the German chemist Viktor Meyer first applied Grbe's nomenclature to benzene.[9] The current nomenclature was introduced by the Chemical Society in 1879.[10]

These terms can also be used in six-membered heterocyclic aromatic systems such as pyridine, where the nitrogen atom is considered one of the substituents. For example, nicotinamide and niacin, shown meta substitutions on a pyridine ring, while the cation of pralidoxime is an ortho isomer.

However, the first place to start is that it has to do with the stability of the carbocation intermediate in electrophilic aromatic substitution reactions. [See this previous post on the mechanism of electrophilic aromatic substitution]. More specifically, how does each substituent affect the stability of that intermediate?

that is really a wonderful article but something confuses me that what is the trend of halogens when they behave as deactivating ortho para directors which halogen is most de activating and which is least next i want to ask that which is more activating OR or OH

How does the pathway if we want to turning -CHO as a meta director into ortho and para director? For example if we want to substitute a hydroxyl group to the position of para or ortho on cinnamaldehyde. thans for ur answer.

Substitution reactions to install hydroxy groups are very difficult. In theory you could nitrate, reduce to NH2, convert to diazonium, and then KOH, but in practice, you could start with benzaldehyde and install a temporary directing group for directed ortho lithiation followed by an oxygen electrophile

Hi James, thank you for the brilliant article?

What if we have a trityl as a substituent? I believe it should be a meta director since the carbon would be quite electrophilic due to the -I effect of the phenyl group. Thoughts?

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Electrophilic aromatic substitution as one of the most fundamental chemical processes is affected by atoms or groups already attached to the aromatic ring. The groups that promote substitution at the ortho/para or meta positions are, respectively, called ortho/para and meta directing groups, which are often characterized by their capability to donate electrons to or withdraw electrons from the ring. Though resonance and inductive effects have been employed in textbooks to explain this phenomenon, no satisfactory quantitative interpretation is available in the literature. Here, based on the theoretical framework we recently established in density functional reactivity theory (DFRT), where electrophilicity and nucleophilicity are simultaneously quantified by the Hirshfeld charge, the nature of ortho/para and meta group directing is systematically investigated for a total of 85 systems. We find that regioselectivity of electrophilic attacks is determined by the Hirshfeld charge distribution on the aromatic ring. Ortho/para directing groups have most negative charges on the ortho/para positions, while meta directing groups often possess the largest negative charge on the meta position. Our results do not support that ortho/para directing groups are electron donors and meta directing groups are electron acceptors. Most neutral species we studied here are electron withdrawal in nature. Anionic systems are always electron donors. There are also electron donors serving as meta directing groups. We predicted ortho/para and meta group directing behaviors for a list of groups whose regioselectivity is previously unknown. In addition, strong linear correlations between the Hirshfeld charge and the highest occupied molecular orbital have been observed, providing the first link between the frontier molecular orbital theory and DFRT.

Therefore, depending on the character of the initial substituent (R), a subsequent substituent would be placed at the ortho or para position if R is an activator/halogen or at the meta position if it is a deactivator (but not a halogen).

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Ortho, Meta and Para refer to the relationship between substituents on a disubstituted benzene ring. In the context of Electrophilic Aromatic Substitution, understanding the chemistry of substituents will help you figure out where to direct the incoming electrophile on a substituted benzene ring.

Not quite!

WHERE you place the incoming electrophile is directly influenced by the chemical nature of the substituent(s) already on the benzene ring. Initial substituents act as directors for the incoming groups.

Deactivating Group

A group already on benzene that deactivates the ring towards Electrophilic Aromatic Substitution reactions. A group that will slow down (and sometimes completely halt) the EAS reaction.

Simply put, electron donating groups are activators and therefore act as ortho and para directors. Electron withdrawing groups are deactivators and therefore act as meta directors. (Halogens are the annoying exception.)

Since these negative groups are happy to sit directly near a carbocation, and since the carbocation forms AT their position only in ortho and para addition, activating groups force incoming electrophiles to add to the ortho or para position.

Different reactions will have different percentages for ortho and para positions. This has to be evaluated in a lab setting and so you will NOT be expected to differentiate. The one exception is to know that a bulky substituent will force para addition simply due to the steric hindrance of its bulk.

As such, electron withdrawing groups slow down, thus deactivating, the EAS reaction. And when they finally agree to react (slowly), these groups want to be as far away from the positive charge as possible.

Ortho, Meta and Para refer to the 1-2, 1-3, and 1-4 relationships between benzene substituents. In Electrophilic Aromatic Substitution reactions, O/M/P directing effects help us figure out where to place the incoming electrophile. Electron Donating Groups activate the ring for ortho and para addition. Electron Withdrawing Groups deactivate the ring for meta addition. Halogens are the one exception.

The letters o, m, and p have been used in place of ortho, meta, and para, respectively, to designate the 1,2-, 1,3-, and 1,4- isomers of disubstituted benzene. This usage is strongly discouraged and is not used in preferred IUPAC names.

The prefix "ortho-" means straight or right; "meta-" means beyond or after; "para-" means beside or along. How, then, did ortho-, meta- and para- come to refer to the carbon positions one, two, and three positions away from a reference point on a benzene?

If we have two groups on a benzene ring immediately beside one another, we do not denote the positions by para-, which would seem logical given its meaning, but instead ortho-. For instance, a benzene ring with two hydroxyl (-OH) groups on immediate positions of a benzene ring can be called para-hydroxybenzene (with the common name "quinone"). What is the etymological history that led to the rather counterintuitive use of these prefixes in organic chemistry?

Wilhelm Krner between 1866-1874 was the first to use it to differentiate isomers of benzene rings. Since the Greek suffixes are fairly vague he chose them arbitrary. Its interesting he used orth- for 1,4 isomer, meta- for the 1,2 isomer , and pera- for the 1,3 isomer. While later chemists used orth- for 1,2 isomer, meta- for the 1,3 isomer , and pera- for the 1,4 isomer. Then in 1879 the Chemical Society of London officially adopted the backwards notation.

Monosubstituted rings will have 5 protons in the region 6.5-8.5 ppm; disubstituted rings will have 4 protons; trisubstituted rings will have 3 protons (and so on). Examples of the NMR of aromatics of mono-, di-, and tri-substituted aromatics are shown below. When interpreting the spectrum of an aromatic compound, remember to count the number of protons in the aromatic region to determine how many times the ring is substituted.

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