Reactivity


Also found in: Dictionary, Thesaurus, Medical, Legal, Wikipedia.
Related to Reactivity: Reactivity series

reactivity

[‚rē·ak′tiv·əd·ē]
(chemistry)
The relative capacity of an atom, molecule, or radical to combine chemically with another atom, molecule, or radical.
(nucleonics)
A measure of the deviation of a nuclear reactor from the critical state at any instant of time such that positive and negative values correspond to reactors above and below critical, respectively; measured in percent k, millikays, dollars, or in-hours.

Reactivity

 

a measure of the chemical activity of substances accounting for the variety and rate of the reactions possible for a given substance. For example, noble metals (Au, Pt) and inert gases (He, Ar, Kr, Xe) are chemically inactive; that is, they possess low reactivity. Alkali metals (Li, Na, K, Cs) and halogens (F, Cl, Br, I), on the other hand, are chemically active; that is, they possess high reactivity.

In organic chemistry, alkanes are characterized by low reactivity and therefore can undergo only reactions (radical halo-genation and nitration, dehydrogenation, dissociation with rupture of C—C bond) occurring under such vigorous conditions as elevated temperatures and ultraviolet irradiation. For the alkyl halides, additional reactions, such as dehydrohalogen-ation, nucleophilic substitution for the halogen, and the formation of organomagnesium compounds, are possible under mild conditions. The presence of double or triple bonds or functional groups (hydroxyl —OH, carboxyl —COOH, amino —NH2) in a molecule promotes a further increase in reactivity.

Reactivity is quantitatively expressed by specific rate constants or, in the case of reversible processes, by equilibrium constants. Modern concepts of reactivity are based on the valence-bond theory and on the study of the distribution (and displacement upon action of a reagent) of the electron density in a molecule. Electron displacements are defined qualitatively through inductive and mesomeric effects and quantitatively through quantum-mechanical calculations. The key factor determining the relative reactivity of a number of related compounds is the molecular structure, which encompasses the nature of the substituents, the electronic and steric effect of these substituents on the reaction center, and the geometry of the molecule. Reactivity is dependent on the reaction conditions, for example, the nature of the medium, presence of catalysts or inhibitors, pressure, temperature, and irradiation. Depending on the mechanism of a given reaction, these factors all have a varied and sometimes contrary effect on the reaction rate. The quantitative relationship between rate (or equilibrium) constants within one reaction series may be represented by correlation equations, which describe the change in the constant as a function of the change in a given parameter. Examples are seen in the Hammett-Taft equation, which deals with the effect of a substituent, and the Brønsted equation, which deals with solvent polarity.

References in periodicals archive ?
Patients were followed for one year to determine the relationship between platelet reactivity and subsequent events.
Only when the juice is contaminated does it cause immune reactivity with alpha-gliadin antibody.
The offering of platelet reactivity testing is a compelling example of laboratory leadership driving improvements that benefit patients.
Also considering the recent inclusion of platelet reactivity testing in both the ACCF/AHA UA/NSTEMI Guidelines and the STS/SCA Blood Conservation Guidelines, the VerifyNow System is making a significant impact on how physicians are treating their cardiovascular patients.
Rather than prescribing a fixed, higher dose of clopidogrel based on a single post-PCI platelet function test, we need to consider alternative treatment strategies that incorporate platelet function testing, such as using more potent antiplatelet agents or repeated testing to a specific target of reactivity.
The Reactivity SDK is just the tool we need to help us perform single sign-on integration between our existing applications and our partners' applications, enabling us to create the custom data transformations ourselves and execute them in the Reactivity Gateway," said Kim Olson, CMG data architect, at Carlson Marketing Group.
With the Reactivity XML Firewall installed, it is necessary only to provision the service to the XML Firewall, which takes only minutes, and then the Reactivity XML Firewall handles everything else.
Immunoreactivity with the monoclonal antibody against human galectin-3 showed strong staining in the majority of malignant lesions and only focal reactivity in a few benign adenomatous lesions (Figure 1).
Once the different antigens have been evaluated for reactivity in this manner, a treatment vial can be mixed that will include each clinically pertinent antigen at its individual endpoint concentration.
When this incidence is compared with the overall incidence of allergic reactions to cephalosporins (4%), there is a 2-fold increase of reactivity in patients allergic to penicillins (4).
T-cell reactivity of the patients' peripheral blood mononuclear cells against donor spleen cells were tested immediately before the medication change.
This is the sign for a different reactivity of the product's crosslinking system and is riot to be considered as a deficiency, but rather as the proof that this system is adjusted to less reactive elastomers.