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The '''mesomeric effect''' or '''resonance effect''' in [[chemistry]] is a property of [[substituent]]s or [[functional group]]s in a [[chemical compound]]. The effect is used in a qualitative way and describes the electron withdrawing or releasing properties of substituents based on relevant [[resonance structure]]s and is symbolized by the letter '''M'''. The mesomeric effect is negative ('''-M''') when the substituent is an electron-withdrawing group and the effect is positive ('''+M''') when based on resonance the substituent is an electron releasing group.
{| border=1 align="right" cellpadding=2 cellspacing=0 width=250 style="margin-left:3em; margin-bottom: 2em; color: black; background: lightyellow;"
|colspan=2|[[Image:Rover.80.bristol.750pix.jpg|250px|1962 Rover P4 80]]
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!colspan=2 style="color: white; background: green;"|Rover P4
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|width="40%"|Manufacturer:||[[British Leyland]]
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|Production:||[[1949]]–[[1964]]
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|Successor:||[[Rover P3]]
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|Successor:||[[Rover P6]]
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|Class:||[[FR layout|FR]] [[midsize car]]
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!colspan=2 style="color: white; background: green;"|P4 75
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|Production:||[[1949]]–[[1954]]<br>43,241 produced
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|Body Styles:||4-door [[sedan (car)|saloon]]
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|Engines:||2.1 L ''[[Rover IOE engine|IOE]]'' [[I6]]
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!colspan=2 style="color: white; background: green;"|P4 60
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|Production:||[[1953]]–[[1959]]<br>9,666 produced
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|Body Styles:||4-door [[sedan (car)|saloon]]
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|Engines:||2.0 L [[I4]]
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!colspan=2 style="color: white; background: green;"|P4 90
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|Production:||[[1953]]–[[1959]]<br>9,666 produced
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|Body Styles:||4-door [[sedan (car)|saloon]]
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|Engines:||2.3 L ''[[Rover IOE engine|IOE]]'' [[I6]]
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!colspan=2 style="color: white; background: green;"|P4 75 Mk. II
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|Production:||[[1954]]–[[1959]]
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|Body Styles:||4-door [[sedan (car)|saloon]]
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|Engines:||2.1 L ''[[Rover IOE engine|IOE]]'' [[I6]]<br>2.2 L ''[[Rover IOE engine|IOE]]'' [[I6]]
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!colspan=2 style="color: white; background: green;"|P4 105R/105S
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|Production:||[[1956]]–[[1959]]<br>10,781 produced
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|Body Styles:||4-door [[sedan (car)|saloon]]
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|Engines:||2.6 L ''[[Rover IOE engine|IOE]]'' [[I6]]
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!colspan=2 style="color: white; background: green;"|P4 80
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|Production:||[[1959]]–[[1962]]<br>5,900 produced
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|Body Styles:||4-door [[sedan (car)|saloon]]
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|Engines:||2.3 L [[I4]]
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!colspan=2 style="color: white; background: green;"|P4 100
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|Production:||[[1960]]–[[1962]]<br>16,521 produced
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|Body Styles:||4-door [[sedan (car)|saloon]]
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|Engines:||2.6 L ''[[Rover IOE engine|IOE]]'' [[I6]]
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!colspan=2 style="color: white; background: green;"|P4 95/110
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|Production:||[[1962]]–[[1964]]<br>4,620 produced
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|Body Styles:||4-door [[sedan (car)|saloon]]
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|Engines:||2.6 L ''[[Rover IOE engine|IOE]]'' [[I6]]
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*Examples of -M substituents: [[acetyl]] (UPAC ethanoyl) - [[nitrile]] - [[nitro]]
The '''[[Rover (car)|Rover]] P4''' series was a group of [[sedan (car)|saloon]] [[automobile]]s produced from [[1949]] through [[1964]]. Along with the later [[Rover P5]], it was replaced by the far more modern [[Rover P6]]. All P4s were 4-door [[sedan (car)|saloons]] with seating for four.
*Examples of +M substituents: [[alcohol]] - [[amine]]
The net electron flow from or to the substituent is determined also by the [[inductive effect]]. It's important to note that the mesomeric effect as a result of p - orbital overlap (resonance) has absolutely no effect on this inductive effect, as the inductive effect is purely to do with the [[electronegativity]] of the atoms and their structural chemistry (which atoms are connected to which).
==P4 75==
The concepts of mesomeric effect, '''mesomerism''' and '''mesomer''' were introduced by [[Christopher Kelk Ingold|Ingold]] in 1938 as an alternative to the [[Linus Pauling|Pauling's]] synonymous concept of [[resonance effect|resonance]] <ref>''If It's Resonance, What Is Resonating?'' Kerber, Robert C. . J. Chem. Educ. '''2006''' 83 223. [http://www.jce.divched.org/Journal/Issues/2006/Feb/abs223.html Abstract]</ref>. Mesomerism is in this context is often encountered in German and French literature but in English literature the term resonance dominates.
The original P4, the model 75, arrived in [[1949]]. It featured controversial modern styling which contrasted with the outdated [[Rover P3]] which it replaced. One odd feature was the centrally-mounted headlight in the grille. Known as the "[[Cyclops]] eye", it was removed after [[1952]]. All P4s featured aluminium body panels until the final 95/110 models.
== External links==
Power came from a 2.1 L (2103 cc/128 in³) ''[[Rover IOE engine|IOE]]'' [[straight-6]] engine. This engine (taken from the [[1948]] P3) was half modern, with [[overhead valve]]s for [[intake manifold|inlet]] and [[side-valve|side valves]] for [[exhaust manifold|exhaust]]. A four-speed [[manual transmission]] was used with a column-mounted shifter at first and floor-mounted unit later. Production of this original model ended in [[1954]] with 43,241 sold.
* IUPAC [[Gold Book]] [http://www.iupac.org/goldbook/M03844.pdf definition]
==P4 60See also ==
* [[List of publications in chemistry#Organic chemistry|Important publications in organic chemistry]]
== References ==
A [[straight-4|four-cylinder]] version of the P4 line was introduced in [[1953]] as the '''P4 60'''. Its 2.0 L (1997 cc/121 in³) engine was the same one Rover used in the [[Land Rover]], but the harsh little engine was out of place in the relatively sumptious P4. It was replaced in [[1959]] by the P4 80, which used an updated version of the Land Rover four.
<references />
[[Category:Chemical bonding]]
==P4 90==
[[de:Mesomerer Effekt]]
At the same time a four cylinder version was introduced, the top-end '''P4 90''' bowed with a more-powerful 2.6 L (2639 cc/161 in³) six. This engine produced a respectable 90 hp (67 kW) and could reach 90 mph (145 km/h). When it was replaced by the P4 100 in [[1959]], 9,666 had been produced.
==P4 75 Mk. II==
The updated P4 75 arrived in [[1954]] with some styling changes. A three-piece wraparound rear window was used, but the 2.1 L (2103 cc/128 in³) IOE engine continued. This model was updated again in [[1955]] with a larger 2.2 L (2230 cc/136 in³) version of the IOE. In [[1957]], it was restyled, along with the rest of the P4 line, with a new grille and fenders. Production ended in [[1959]] with the introduction of the P4 100.
==P4 105R/105S==
The P4 90 was not to be the top of the P4 line, however. Introduced in [[1956]], the '''P4 105R''' and '''P4 105S''' used a high-output version of the 2.6 L (2639 cc/161 in³) engine also used in the 90. This twin-[[carburettor]] engine produced 108 hp (80 kW). Both 105 models also featured an updated exterior and more luxury trim inside.
The 105R featured a "Roverdrive" [[automatic transmission]]. This troublesome unit was actually a two-speed automatic with an [[overdrive]] unit for a total of four forward gears. It could reach 91 mph (147 km/h). The 105S made due with a [[manual transmission]] with overdrive but could hit 101 mph (163 km).
When production of the 105 line ended (in [[1958]] for the 105R and [[1959]] for the 105S), 10,781 had been produced, two thirds with the manual transmission option.
==P4 80==
The four cylinder version of the P4 was replaced in [[1959]] with the '''P4 80'''. It too used a Land Rover-derived [[straight-4]] engine, this time displacing 2.3 L (2286 cc/139 in³). With 80 hp (59 kW) available, the car could top 85 mph (137 km/h). Power [[disc brake]]s at the front were new, and the car used wider tyres and had updated styling. But the four cylinder P4s were never popular, and just 5,900 had been built when the line was cancelled in [[1962]].
==P4 100==
The P4 90 was replaced in [[1960]] by the more-powerful '''P4 100'''. Sharing the same 2.6 L (2625 cc/160 in³) ''[[Rover IOE engine|IOE]]'' straight-6 engine, the car could now reach 100 mph (161 km/h). The interior was luxurious, with wood and leather accents on traditional English car elements like a curved "Shepherds Crook" [[handbrake]] lever. 16,521 had been produced when it was replaced by the 110 version in [[1962]].
==P4 95/110==
The final member of the P4 family was the '''P4 95''' and '''P4 110''' series. Introduced in [[1962]], and looking quite dated by then, the P4 ws later altered to have steel door panels and electric windscreen washers. [[Overdrive]] was standard, though the Roverdrive automatic had been put to rest.
Both models used the same 2.6 L (2625 cc/160 in³) version of the IOE engine. This old unit produced 123 hp (91 kW) in 110 guise, which used a Weslake cylinder head, and 102 hp (76 kW) for the 95. Both were replaced by the futuristic [[Rover P6]] after [[1964]].
==External link==
*[http://www.roverp4.com/ Rover P4 cars enthusiast site]
[[Category:Rover vehicles|P4]]
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