# 23.4: Reactivity of Organic Molecules (2023)

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##### learning goals
• Understand the relationship between the structure and reactivity of a variety of related organic compounds.

To understand why organic molecules react in this way, it is necessary to know something about the structure and properties of the short-lived species that are formed in chemical reactions. Identification of transient intermediates allows chemists to elucidate reaction mechanisms, often allowing them to control the products of a reaction. When designing the synthesis of a molecule, such as a new drug, chemists must be able to understand the mechanisms of intermediate reactions to maximize the yield of the desired product and minimize the occurrence of undesirable reactions. Also, by recognizing the common reaction mechanisms of simple organic molecules, we can understand how more complex systems react, including the much larger molecules found in biochemistry.

Almost all chemical reactions, whether organic or inorganic, take place because positively or partially positively charged atoms or groups of atoms interact with negatively or partially negatively charged atoms or groups of atoms. Therefore, when a bond in a hydrocarbon is broken during a reaction, identifying the transient species formed, some of which are charged, allows chemists to determine the mechanism and predict the products of a reaction.

(Video) Representing structures of organic molecules | Biology | Khan Academy

Chemists often find that the reactivity of a molecule is affected by the degree of substitution of a carbon attached to a functional group. These carbons are called primary, secondary, or tertiary. A primary carbon is attached to another carbon and a functional group, a secondary carbon is attached to two other carbons and a functional group, and a tertiary carbon is attached to three other carbons and a functional group.

## reactive intermediates

Breaking a C-H bond can generate -C+wed H, -C y H o -Cwed H+, all of which are unstable and therefore highly reactive. The most commonly formed species is –C+, what's it calledcarbocatião(Part (a) in Figure 24.3.1). A carbocation has only six valence electrons and is therefore electron poor. It's aelectrophile(from "electron" and the Greek suffix phile, meaning "to love"), a species that needs electrons to complete its octet. (Remember that electron-poor compounds such as those ofGroup 13 elements, act as Lewis acids in inorganic reactions.) In general, when a strongly electronegative atom such as Cl binds to a carbocation, it pulls electrons from carbon and destabilizes the positive charge. Conversely, alkyl groups and other species stabilize the positive charge by increasing the electron density in the carbocation. Therefore, a tertiary carbocation (R3C+) is more stable than a primary carbocation (RCH2+).

The reactivity of a molecule is often affected by the degree of substitution of the carbon attached to a functional group.

Adding an electron to a carbocation creates a neutral species called a radical. An example is the methyl radical (CH3), shown in part (b) of Figure 24.3.1. Since carbon still has less than an octet of electrons, it is electron poor and also behaves like an electrophile. Like carbocations, radicals can be stabilized by carbon substituents, which can donate some electron density to the electron-poor carbon center. Like carbocations, a tertiary radical (R3C) is more stable than a primary radical (RCH2·).

Adding an electron to a radical creates a carbanion that contains a negatively charged carbon with eight valence electrons (part (c) in Figure 24.3.1). The methyl anion (CH3) has a structure similar to NH3with its lone pair of electrons, but it has a much stronger tendency to share its lone pair of electrons with another atom or molecule. A carbanion is a nucleophile (from "core" and "edge"), an electron-rich species that has an available pair of electrons that it can share with another atom. Carbanions are destabilized by electron-donating groups, so the relationship between their structure and reactivity is exactly the opposite of that for carbocations and radicals. That is, a tertiary carbanion (R3C) is less stable than a primary carbanion (RCH2). Carbanions are most commonly found in organometallic compounds such as methyllithium (CH3Li) or methylmagnesium chloride (CH3MgCl), with the more electropositive metal ion stabilizing the negative charge on the more electronegative carbon atom.

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Electrophiles, like carbocations, seek to accept electrons and therefore have a strong tendency to react with nucleophiles, which are negatively charged species or substances with lone pairs of electrons. The reaction of electrophiles with nucleophiles is a central issue in organic reactions.

Electrophiles react with nucleophiles.

##### Example $$\PageIndex{1}$$

Classify each species as electrophilic, nucleophile, or neither.

1. Freund3
2. CH4
3. (CH3)3C+
4. N.H2

given: molecular formulas

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Solicitous: reactivity mode

Strategy:

Determine if the compound is electron deficient, in which case it is an electrophile; rich in electrons, in this case it is a nucleophile; Or none.

Solution:

1. the boyfriend3The molecule is a neutral compound containing a group 13 element with three bonds to B. The boron atom only has six valence electrons, so it tends to accept a pair of electrons. Therefore, the compound is an electrophile.
2. the ONLY4The molecule has four C bonds, which is typical of a neutral Group 14 compound. The carbon atom has no lone pairs of electrons to share and does not tend to gain electrons, and each hydrogen atom forms a bond containing two electrons. each. other. so CH4it is neither an electrophile nor a nucleophile.
3. or only3)3C+the cation contains a Group 14 (carbon) atom with only three bonds. Therefore, it only has six valence electrons and is looking for electrons to complete an octet. Therefore (CH3)3C+, a carbocation, is an electrophile.
4. or SMALL2The anion contains a Group 15 element with a lone pair of electrons, two bonds, and a negative charge, giving N a total of eight electrons. With its negative charge, the N atom has two lone pairs of electrons, making it a powerful nucleophile.
##### Exercise $$\PageIndex{1}$$

Classify each compound as electrophilic, nucleophile, or neither.

1. C6H5OH
2. AlBr3
3. (CH3)4C

respondedor:

1. nucleophile
2. electrophile
3. none
(Video) The Molecules of Life

## Summary

Electrophiles have a strong tendency to react with nucleophiles. The reactivity of a molecule is often affected by the degree of substitution of the carbon attached to a functional group; Carbon is called primary, secondary, or tertiary. Identifying the transient species formed in a chemical reaction, some of which are charged, allows chemists to predict the mechanism and products of the reaction. A common transient species is a carbocation, a carbon with six valence electrons that is an electrophile; that is, electrons are needed to complete its octet. A radical is a transient species that is neutral but electron deficient and therefore acts as an electrophile. By contrast, a carbanion has eight valence electrons and is negatively charged. It is an electron-rich species that is a nucleophile because it can share an electron pair. In chemical reactions, electrophiles react with nucleophiles.

## conceptual problems

1. organize CH2F+, I WANT2+, CH3+, e CHF2+to increase stability. Explain your reasoning.
2. organize CH3CH2+, CHBR2+, CH3+, e CHBrCl+in descending order of stability. Explain your reasoning.
3. Identify the electrophile and the nucleophile in each pair.
1. CH3it's him+
2. CH3ONa and formaldehyde
3. H+and propylene
4. Benzol y Cl
1. Identify the electrophile and the nucleophile in each pair.
1. CH3+my brother
2. HC≡CNa y pentanal
3. acetone and CN
4. (CH3)2S e CH3UE

1. Swiss francs2+<CHCl2+<CH2F+<CH3+; Electronegative substituents destabilize the positive charge. The greater the number of electronegative substituents and the greater their electronegativity, the more unstable the carbocation.
2. CH3o, nucleophile; formaldehyde, electrophile
3. H+, electrophile; Propylene Nucleophile
4. benzene, electrophile; Cl, nucleophile
(Video) Biological Molecules - You Are What You Eat: Crash Course Biology #3

## structure and reactivity

1. Draw the Lewis electronic structures of the products of the carbon-hydrogen cleavage reactions. What is the charge of each type?
2. Identify the electrophile and the nucleophile in each reaction; Then complete each chemical equation.
1. CH3++Kl
2. CH3CH=CH2+ HBr →
3. (CH3)3N+BCl3

## FAQs

### What is rate of reactivity of organic compounds? ›

The rate of reactivity of these compounds is slow due to the presence of covalent bonds. Covalent bonds are strong bonds which are formed by sharing of electrons and high energy is needed to break these bonds so as to facilitate the reactions of organic compounds.

What determines the chemical reactivity of organic molecules? ›

The reactivity of the organic molecule is primarily dependent upon the attached functional group such as a hydroxyl group. Chemical properties are also linked with the type of functional group that is present.

How do you find the order of reactivity? ›

Reactivity series is a list of metals arranged in decreasing order of their reactivity. Most reactive metals are at the top while the least reactive metals at the bottom. For any two metals in the series, the metal placed higher in the series can displace lower metals from their salt solution.

How do you know if a molecule is more reactive? ›

The number of electrons in the outermost shell of an atom determines its reactivity.

Does higher reactivity mean faster reaction? ›

Reactant Stability/Reactivity: The more stable the reactant, the less reactive it will be. In terms of rates, this means that the more stable the reactant, the slower it will react.

What is reactivity in organic molecules? ›

The reactivity of a molecule is often affected by the degree of substitution of the carbon bonded to a functional group. Adding one electron to a carbocation produces a neutral species called a radical. An example is the methyl radical (·CH3), shown in part (b) in Figure 24.3.

What 3 factors determine the reactivity of elements? ›

Four major factors affect reactivity of metals: nuclear charge, atomic radius, shielding effect and sublevel arrangement (of electrons).

How do you determine the rate of reaction in organic chemistry? ›

During the course of the reaction shown below, reactants A and B are consumed while the concentration of product AB increases. The reaction rate can be determined by measuring how fast the concentration of A or B decreases, or by how fast the concentration of AB increases.

What is the trend of reactivity series? ›

Chemical reactivity of the elements

Period - reactivity decreases as you go from left to right across a period. The farther to the left and down the periodic chart you go, the easier it is for electrons to be given or taken away, resulting in higher reactivity.

What is the degree of reactivity? ›

In general, the reactivity definition is the degree to which a substance shows chemical change when mixed with another substance. It is a measurement of how much a substance reacts with others. The scientific study of chemical changes and their kinetics is another form of reactivity definition.

### What is meant by order of reactivity? ›

Definition. The Order of Reaction refers to the power dependence of the rate on the concentration of each reactant. Thus, for a first-order reaction, the rate is dependent on the concentration of a single species.

What does it mean for a substance to have a high reactivity? ›

Reactive chemicals are defined as those substances which can, in contact with air, water or other common substances, vigorously or violently give off heat, energy or toxic gases or vapors.

What molecule is most reactive? ›

Fluorine: The Most Reactive And Indispensable Chemical Element In Our Daily Lives. Fluorine is a remarkable chemical element - not only because of its unique reactivity.

What makes a substance highly reactive? ›

Highly reactive chemicals include those that have the potential to vigorously polymerize, undergo a vigorous condensation or oxidation-reduction reaction, or become self-reactive due to shock, pressure, temperature, light, or contact with another material.

Does reactivity increase or decrease down a group? ›

Hence, down the group reactivity increases.

Does reactivity increase when going down a group? ›

Reactivity of elements increases down the group as down the group number of shells increases and thus nuclear pull on the outermost electrons decreases.

Does reactivity increase down a group? ›

As we go down a group the reactivity of non-metals decrease. This is because : The number of valence shells increases. The force with which the nucleus holds the electrons decreases.

Which organic compound is more reactive? ›

EXPLANATION: Ethyne consists of a triple bond that is most easier to be broken as compared to that of ethyne with a double bond and benzene single bonds. Therefore ethyne is most reactive among the given options.

Which elements has the highest reactivity? ›

The elements toward the bottom left corner of the periodic table are the metals that are the most active in the sense of being the most reactive. Lithium, sodium, and potassium all react with water, for example.

What is a good example of reactivity? ›

Reactivity is the ability of matter to combine chemically with other substances. For example, iron is highly reactive with oxygen. When it combines with oxygen, it forms the reddish powder called rust (see Figure below). Rust is not iron but an entirely different substance that consists of both iron and oxygen.

### Why is reactivity so important? ›

The author concludes that without chemical reactivity, there would be no hazardous characteristics such as toxicity, ignitability, corrosivity, or flammability exhibited by a substance. Chemical reactivity enhances the hazardous nature of a substance.

Which factors will increase the reactivity of group? ›

(i) Presence of a group with a positive inductive effect. (ii) Presence of a group with a negative inductive effect. (iii) Presence of large alkyl group.

Does reactivity increase? ›

The greater the distance between the nucleus of the atom and the valence electron, the lower the effective nuclear charge on the electron and the greater the reactivity of the atom. Therefore, the reactivity of elements increases down a group as the atomic size increases.

What Period 3 elements are highly reactive? ›

these elements are Sodium (Na), Magnesium(Mg), Aluminum (Al), Chlorine (Cl), silicon (Si), Phosphorus (P), Sulphur (S) and argon (Ar).

Why is 3rd degree more reactive? ›

A tertiary carbocation is more stable than a secondary carbocation which is more stable than a primary carbocation. Greater the stability of the carbocation, greater will be the ease of formation of carbocation, and hence faster will be the rate of the reaction.

What are the properties of reactivity? ›

Reactivity is the ability of matter to combine chemically with other substances. Some kinds of matter are extremely reactive; others are extremely unreactive. For example, potassium is very reactive, even with water. When a pea-sized piece of potassium is added to a small amount of water, it reacts explosively.

How do you interpret reaction rates? ›

To measure reaction rates, chemists initiate the reaction, measure the concentration of the reactant or product at different times as the reaction progresses, perhaps plot the concentration as a function of time on a graph, and then calculate the change in the concentration per unit time.

What are the 5 rates of reaction? ›

There are five general properties that can affect the rate of a reaction:
• The concentration of the reactants. The more concentrated the faster the rate.
• Temperature. ...
• Physical state of reactants. ...
• The presence (and concentration/physical form) of a catalyst (or inhibitor). ...
• Light.

What are 2 ways to measure calculate reaction rates? ›

There are two suggested ways to measure rate of reaction when changing concentration. The first is to measure how quickly a precipitate is formed by observing the 'cloudiness' of a liquid. The second is to measure the volume of gas produced using a gas syringe or a measuring cylinder over water.

What is the increasing order of reactivity series? ›

The arrangement of metals as per reactivity series is: K > Na > Ca > Mg > Al > Zn > Fe > Ni > Sn > Pb > H > Cu > Ag > Au. Therefore, the correct order is Fe > Ni > Sn > Pb > H > Cu > Au.

### Why is the reactivity series in that order? ›

The reactivity series of metals is a chart listing metals in order of decreasing reactivity. In general, the more reactive a metal is: the more vigorously it reacts with other substances. the more easily it loses electrons to form positive ions (cations)

What are dangerously reactive material examples? ›

Most of these materials can be extremely hazardous if they are not handled properly because they can react in such a quick manner very easily. Examples of these products are ethyl acrylate, vinyl chloride, ethylene oxide, picric acid and anhydrous aluminum chloride.

What are the 5 most reactive elements? ›

The reactivity series follows the order, from most reactive to least reactive:
• Cesium.
• Francium.
• Rubidium.
• Potassium.
• Sodium.
• Lithium.
• Barium.
Feb 10, 2020

Which groups are highly reactive? ›

The two most reactive groups of elements are the alkali metals and the halogens, because of their valence electrons.

What atoms are highly reactive? ›

Halogens are highly reactive because they readily gain an electron to fill their outermost shell. Alkali metals are highly reactive because they readily lose the single electron in their outermost shell.

Do organic compounds have fast reaction rates? ›

Explanation: By and large, organic compounds contain mainly covalent bonds. Covalent bonds are typically harder to break, which is why organic reactions happen at a relatively slower rate than inorganic reactions.

What is the rate law in organic chemistry? ›

A rate law shows how the rate of a chemical reaction depends on reactant concentration. For a reaction such as aA → products, the rate law generally has the form rate = k[A]ⁿ, where k is a proportionality constant called the rate constant and n is the order of the reaction with respect to A.

Are organic compounds less reactive? ›

Reactions involving organic compounds are generally slower than inorganic reactions. Because of strong covalent bonding within the molecule, organic compounds do not readily react.

How do you know which reaction is faster in organic chemistry? ›

The frequency of collisions: The more often molecules collide with each other, the faster the reaction proceeds. The energy of collisions: The more forcefully molecules collide with each other, the more likely they are to react, and the faster the reaction proceeds.

What organic molecules speed up reactions? ›

The answer is enzymes. Enzymes in our bodies are catalysts that speed up reactions by helping to lower the activation energy needed to start a reaction. Each enzyme molecule has a special place called the active site where another molecule, called the substrate, fits.

### How do you know which reaction is the fastest? ›

If the phase of matter (solid, liquid, or gas) of the reactants results in more collisions, the reaction will be faster. Gas molecules move fast, so gases usually react faster than liquids. Liquids move faster than solids, so liquids react faster than solids.

What is the most reactive molecule? ›

Answer: Cesium is the most reactive element since it is the second from the bottom of this group, has six electron shells, and exhibits the characteristics of a reactive atom.

What is the highest reactivity? ›

Caesium, the most reactive metal in the periodic table, reacts extremely violently – hence why it can't be demonstrated in a classroom! This can be compared to other common metals, such as iron and copper, which produce no reaction when dropped into water.

How do you calculate rate of reaction in organic chemistry? ›

During the course of the reaction shown below, reactants A and B are consumed while the concentration of product AB increases. The reaction rate can be determined by measuring how fast the concentration of A or B decreases, or by how fast the concentration of AB increases.

What is order of the reaction in organic chemistry? ›

The Order of reaction refers to the relationship between the rate of a chemical reaction and the concentration of the species taking part in it. In order to obtain the reaction order, the rate expression (or the rate equation) of the reaction in question must be obtained.

What is rate of a chemical reaction? ›

The rate of a chemical reaction is defined as the rate of change in concentration of a reactant or product divided by its coefficient from the balanced equation.

What is the least reactive organic compound? ›

The alkanes are the least reactive organic compounds because they lack functional groups and, as a consequence, of polarized bonds.

What is the least reactive molecule? ›

Noble gases are the least reactive of all elements. That's because they have eight valence electrons, which fill their outer energy level. This is the most stable arrangement of electrons, so noble gases rarely react with other elements and form compounds.

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