An ionic bond is a bond that results from the electrostatic attraction force between ions of opposite charges. Ionic bonds apply to ionic compound, such as sodium chloride NaCl. In simple ionic compounds, the metal element loses valence electron s to form the cation and the non-metal element gains electron s to form the anion.
This is also called the octet rule. A covalent bond is a bond formed through the sharing of electron pairs between the two bonding atoms. The shared electron pairs are mutually attracted by the nuclei of both atoms.
By sharing the electron pairs, both atoms also gain a filled outer shell, or an octet. Almost all of the bonds involved in organic compounds are covalent bonds. One orbital of 2s and two orbitals of 2p mix up forming three hybrid orbitals of equivalent energy, while the third p orbital remains unhybridized. The three new equivalent orbitals are called sp 2 hybrid orbitals.
Three sp 2 hybridised orbitals formed, repel each other and they are directed in a plane towards the three corners of an equilateral triangle. The unhybridised p z orbital remains perpendicular to this plane. Each sp 2 hybrid orbital contains an unpaired electron. In each sp 2 hybrid orbital, one of the lobes is bigger because of more concentration of electron density.
Only bigger lobe is involved in bond formation. As all the six atoms in the C 2 H 4 molecule being in the same plane, the molecule is planar. The unhybridised 2 p z orbitals of each carbon atom being perpendicular to the plane of four hydrogen atoms and carbon atoms overlap laterally with one another to form a week pi bond between two carbon atoms by p-p overlap.
This bond consists of two equal electron cloud one lying above the plane of the atom and other lying below this plane. In acetylene, there is sp hybridisation of carbon atom.
One 2s orbital and one 2p orbital of carbon mix up forming two hybrid orbitals of equivalent energy. These two new equivalent orbitals are called sp hybrid orbitals. How to Predict Type of Hybridization of Carbon:. Example — 1: To find hybridization state of each carbon atom in the following molecule.
Carbon 1 is attached to three other atoms. Hence it is sp 2 hybridized. Carbon 2 is attached to four other atoms. Hence it is sp 3 hybridized. Each carbon is attached to three other atoms. The bond made by electron sharing is called a covalent bond. Covalent bonding and covalent compounds will be discussed in detail below.
NOTE: Despite our focus on the octet rule, we must remember that for small atoms, such as hydrogen, helium, and lithium, the first shell is, or becomes, the outermost shell and hold only two electrons. Covalent bonds have certain characteristics that depend on the identities of the atoms participating in the bond.
Two characteristics are bond length and bond polarity. The covalent bond in the hydrogen molecule H 2 has a certain length about 7. Other covalent bonds also have known bond lengths, which are dependent on both the identities of the atoms in the bond and whether the bonds are single, double, or triple bonds.
Table 1. The exact bond length may vary depending on the identity of the molecule but will be close to the value given in the table. Without exception, as the number of covalent bonds between two atoms increases, the bond length decreases. With more electrons between the two nuclei, the nuclei can get closer together before the internuclear repulsion is strong enough to balance the attraction.
Figure 1. Polar versus nonpolar covalent bonds. This is a nonpolar covalent bond. This is a polar covalent bond. Although we defined covalent bonding as electron sharing, the electrons in a covalent bond are not always shared equally by the two bonded atoms. Unless the bond connects two atoms of the same element, there will always be one atom that attracts the electrons in the bond more strongly than the other atom does, as shown in Figure 1.
A covalent bond that has an unequal sharing of electrons, as in part b of Figure 1. A covalent bond that has an equal sharing of electrons part a is called a nonpolar covalent bond. Any covalent bond between atoms of different elements is a polar bond, but the degree of polarity varies widely. Some bonds between different elements are only minimally polar, while others are strongly polar.
Ionic bonds can be considered the ultimate in polarity, with electrons being transferred rather than shared. To judge the relative polarity of a covalent bond, chemists use electronegativity , which is a relative measure of how strongly an atom attracts electrons when it forms a covalent bond. There are various numerical scales for rating electronegativity. The polarity of a covalent bond can be judged by determining the difference in the electronegativities of the two atoms making the bond.
The greater the difference in electronegativities, the greater the imbalance of electron sharing in the bond. Source: Chirality with hands. Stereoisomers that are not enantiomers, such as glucose and galactose shown above, do have chiral centers and are not superimposable, but they are not mirror images of one another.
Only stereoisomers that are also mirror images and not superimposable are termed enantiomers. Enantiomers are very hard to separate from one another. They are nearly identical in their physical and chemical properties.
They have the same molecular weight, the same polarity, the same melting and boiling points, etc. In fact, enantiomers are so alike that they even share the same name! In Figure 5. Both of the molecules are 2-butanol. But they are not exactly the same molecule, in the same way that your left shoe is not exactly the same as your right.
They are non-superimposable mirror images of each other. How do we communicate this difference? One small difference between enantiomers is the direction that polarized light will rotate when it hits the molecule. One enantiomer will rotate light in the clockwise direction, while the other will rotate it in the counterclockwise direction. However, light rotation cannot be used in a predictive way to determine the absolute stereo-configuration of a molecule i.
Thus, another system is needed to describe the absolute configuration. The Cahn-Ingold-Prelog CIP priority system was designed to determine the absolute stereo-configuration of enantiomers as either sinister S or rectus R.
In this system, the groups that are attached to the chiral carbon are given priority based on their atomic number Z. Atoms with higher atomic number more protons are given higher priority i.
For determining the stereochemistry, place the lowest priority group away from you, so that the other three groups are held are facing you. Assign priority to the remaining groups. The rules for this system of stereochemical nomenclature are, on the surface, fairly simple. Try making a model of the stereoisomer of glyceraldehyde shown below.
Be sure that you are making the correct enantiomer! The first thing that we must do is to assign a priority to each of the four substituents bound to the chiral carbon. In this nomenclature system, the priorities are based on atomic number, with higher atomic numbers having a higher priority.
We first look at the atoms that are directly bonded to the chiral carbon: these are H, O in the hydroxyl , C in the aldehyde , and C in the CH 2 OH group. Two priorities are easy: hydrogen, with an atomic number of 1, is the lowest 4 priority, and the hydroxyl oxygen, with atomic number 8, is priority 1.
Carbon has an atomic number of 6. To determine this, we move one more bond away from the stereocenter: for the aldehyde we have a double bond to an oxygen, while on the CH 2 OH group we have a single bond to an oxygen.
If the atom is the same, double bonds have a higher priority than single bonds. Therefore, the aldehyde group is assigned 2 priority and the CH 2 OH group the 3 priority. With our priorities assigned, we next make sure that the 4 priority group the hydrogen is pointed back away from ourselves, into the plane of the page it is already. Then, we trace a circle defined by the 1, 2, and 3 priority groups, in increasing order. For S -glyceraldehyde, the circle described by the 1, 2, and 3 priority groups is counter-clockwise but first, we must flip the molecule over so that the H is pointing into the plane of the page.
In the case of 2-butanol Fig 5. The -OH is first priority and the -H is fourth priority. How do you assign 2nd and 3rd priority, since both of those atoms are carbon? If the priority is the same for an attached atom, you need to look out to the next level and evaluate priority there. In the first situation, if we look out to the next level, this carbon is bound to three other hydrogen atoms all very low priority.
In the second situation, the carbon is bound to two hydrogens and one carbon. Once all of the groups have been assigned priority, you can determine which direction the priority is moving. In our example, the 2-butanol on the left shows priority moving in the counterclockwise direction giving the S-enantiomer. The molecule on the right shows the R-enantiomer with priority moving in the clockwise direction. The CIP priority system can be used to determine the absolute stereo-conformation of enantiomers.
Interestingly, enantiomers have the same physical properties and exactly the same chemical properties, except when reacting with other chiral molecules. Thus, chiral molecules have potentially drastic differences in physiology and medicine. Thalidomide had previously been used in other countries as an antidepressant, and was believed to be safe and effective.
It was not long, however, before doctors realized that something had gone horribly wrong: many babies born to women who had taken thalidomide during pregnancy suffered from severe birth defects.
Researchers later realized the that problem lay in the fact that thalidomide was being provided as a mixture of two different isomeric forms, called a racemic mixture. One of the isomers is an effective medication, while the other caused the side effects. Both isomeric forms have the same molecular formula and the same atom-to-atom connectivity, so they are not merely structural isomers.
Where they differ is in the arrangement in three-dimensional space about one tetrahedral chiral carbon. Thus, these two forms of thalidomide are enantiomers. Note that the carbon in question has four different substituents two of these just happen to be connected by a ring structure. Tetrahedral carbons with four different substituent groups are called stereocenters.
Looking at the structures of what we are referring to as the two isomers of thalidomide, you may not be entirely convinced that they are actually two different molecules. The two stereoisomers of our simplified model look like this:. Furthermore, they are not superimposable : if we pick up molecule A, flip it around, and place it next to molecule B, we see that the two structures cannot be superimposed on each other.
They are two different molecules! If you make models of the two stereoisomers of thalidomide and do the same thing, you will see that they too are mirror images, and cannot be superimposed it will help to look at a color version of the figure below. Here are some more examples of chiral molecules that exist as pairs of enantiomers.
In each of these examples, there is a single stereocenter, indicated with an arrow. Many molecules have more than one stereocenter, but we will get to that that a little later! Here are some examples of molecules that are achiral not chiral. Notice that none of these molecules has a stereocenter an atom that is bound to four different substituents. Chiral molecules are sometimes drawn without using wedges.
Conversely, wedges may be used on carbons that are not stereocenters — look, for example, at the drawings of glycine and citrate in the figure above. Just because you see dashed and solid wedges in a structure, do not automatically assume that you are looking at a stereocenter.
Other elements in addition to carbon can be stereocenters. The phosphorus center of phosphate ion and organic phosphate esters, for example, is tetrahedral, and thus is potentially a stereocenter. Having trouble visualizing chirality and enantiomers? It may be helpful to watch this. The thalidomide that was used in the s to treat depression and morning sickness was sold as a mixture of both the R and the S enantiomer — this is referred to as a racemic mixture.
The problem with racemic thalidomide, as we learned above, is that only the R enantiomer is an effective medicine, while the S enantiomer causes mutations in the developing fetus. How does such a seemingly trivial structural variation lead to such a dramatic and in this case, tragic difference in biological activity?
Virtually all drugs work by interacting in some way with important proteins in our cells: they may bind to pain receptor proteins to block the transmission of pain signals, for instance, or clog up the active site of an enzyme that is involved in the synthesis of cholesterol. Instead, it seems that S -thalidomide interacts somehow with a protein involved in the development of a growing fetus, eventually causing the observed birth defects.
Source: www. The over-the-counter painkiller ibuprofen is currently sold as a racemic mixture, but only the S enantiomer is effective. Fortunately, the R enantiomer does not produce any dangerous side effects, although its presence does seem to increase the amount of time that it takes for S -ibuprofen to take effect.
You can, with the assistance your instructor, directly experience the biological importance of stereoisomerism. Carvone is a chiral, plant-derived molecule that smells like spearmint in the R form and caraway a spice in the S form. The two enantiomers interact differently with smell receptor proteins in your nose, generating the transmission of different chemical signals to the olfactory center of your brain.
The number of known organic compounds is a quite large. In fact, there are many times more organic compounds known than all the other inorganic compounds discovered so far, about 7 million organic compounds in total.
Fortunately, organic chemicals consist of a relatively few similar parts, combined in different ways, that allow us to predict how a compound we have never seen before may react, by comparing how other molecules containing the same types of parts are known to react. These parts of organic molecules are called functional groups and are made up from specific bonding patterns with the atoms most commonly found in organic molecules C, H, O, N, S, and P. The identification of functional groups and the ability to predict reactivity based on functional group properties is one of the cornerstones of organic chemistry.
Functional groups are specific atoms, ions, or groups of atoms having consistent properties. A functional group makes up part of a larger molecule. For example, -OH, the hydroxyl group that characterizes alcohols, is an oxygen with a hydrogen attached. It could be found on any number of different molecules.
Just as elements have distinctive properties, functional groups have characteristic chemistries. An -OH functional group on one molecule will tend to react similarly, although perhaps not identically, to an -OH on another molecule. Organic reactions usually take place at the functional group, so learning about the reactivities of functional groups will prepare you to understand many other things about organic chemistry. Functional groups are structural units within organic compounds that are defined by specific bonding arrangements between specific atoms.
The structure of capsaicin, the fiery compound found in hot peppers, incorporates several functional groups, labeled in the figure below and explained throughout this section. As we progress in our study of organic chemistry, it will become extremely important to be able to quickly recognize the most common functional groups, because they are the key structural elements that define how organic molecules react.
For now, we will only worry about drawing and recognizing each functional group, as depicted by Lewis and line structures. Much of the remainder of your study of organic chemistry will be taken up with learning about how the different functional groups behave in organic reactions. Below is a brief introduction to the major organic functional groups. Methane, CH 4 , is the natural gas you may burn in your furnace.
Octane, C 8 H 18 , is a component of gasoline. Alkenes sometimes called olefins have carbon-carbon double bonds, and alkynes have carbon-carbon triple bonds. Ethene, the simplest alkene example, is a gas that serves as a cellular signal in fruits to stimulate ripening.
If you want bananas to ripen quickly, put them in a paper bag along with an apple — the apple emits ethene gas also called ethylene , setting off the ripening process in the bananas. Ethyne, commonly called acetylene, is used as a fuel in welding blow torches.
Many alkenes can take two geometric forms: cis or trans. The cis and trans forms of a given alkene are different isomers with different physical properties because there is a very high energy barrier to rotation about a double bond.
In the example below, the difference between cis and trans alkenes is readily apparent. Alkanes, alkenes, and alkynes are all classified as hydrocarbons , because they are composed solely of carbon and hydrogen atoms. The double and triple-bonded carbons in alkenes and alkynes have fewer hydrogen atoms bonded to them — they are thus referred to as unsaturated hydrocarbons.
Aromatic groups are planar flat ring structures, and are widespread in nature. When the carbon of an alkane is bonded to one or more halogens, the group is referred to as an alkyl halide or haloalkane. Chloroform is a useful solvent in the laboratory, and was one of the earlier anesthetic drugs used in surgery.
Chlorodifluoromethane was used as a refrigerant and in aerosol sprays until the late twentieth century, but its use was discontinued after it was found to have harmful effects on the ozone layer. Bromoethane is a simple alkyl halide often used in organic synthesis. Alkyl halides groups are quite rare in biomolecules. In the alcohol functional group, a carbon is single-bonded to an OH group the OH group, when it is part of a larger molecule, is referred to as a hydroxyl group.
Except for methanol, all alcohols can be classified as primary, secondary, or tertiary. In a primary alcohol, the carbon bonded to the OH group is also bonded to only one other carbon. In a secondary alcohol and tertiary alcohol, the carbon is bonded to two or three other carbons, respectively. When the hydroxyl group is directly attached to an aromatic ring, the resulting group is called a phenol.
The sulfur analog of an alcohol is called a thiol from the Greek thio , for sulfur. Note that the definition of a phenol states that the hydroxyl oxygen must be directly attached to one of the carbons of the aromatic ring. The compound below, therefore, is not a phenol — it is a primary alcohol. The distinction is important, because there is a significant difference in the reactivity of alcohols and phenols.
In an ether functional group, an oxygen is bonded to two carbons. Below is the structure of diethyl ether, a common laboratory solvent and also one of the first compounds to be used as an anesthetic during operations. The sulfur analog of an ether is called a thioether or sulfide. Amines are characterized by nitrogen atoms with single bonds to hydrogen and carbon.
Just as there are primary, secondary, and tertiary alcohols, there are primary, secondary, and tertiary amines. Ammonia is a special case with no carbon atoms.
One of the most important properties of amines is that they are basic, and are readily protonated to form ammonium cations. In the case where a nitrogen has four bonds to carbon which is somewhat unusual in biomolecules , it is called a quaternary ammonium ion. In alcohols, what matters is how many other carbons the alcohol carbon is bonded to, while in amines, what matters is how many carbons the nitrogen is bonded to.
Phosphate and its derivative functional groups are ubiquitous in biomolecules. Phosphate linked to a single organic group is called a phosphate ester ; when it has two links to organic groups it is called a phosphate diester. A linkage between two phosphates creates a phosphate anhydride. There are a number of functional groups that contain a carbon-oxygen double bond, which is commonly referred to as a carbonyl.
Ketones and aldehydes are two closely related carbonyl-based functional groups that react in very similar ways. In a ketone , the carbon atom of a carbonyl is bonded to two other carbons. In an aldehyde, the carbonyl carbon is bonded on one side to a hydrogen, and on the other side to a carbon. The exception to this definition is formaldehyde, in which the carbonyl carbon has bonds to two hydrogens.
The main member of this family is the carboxylic acid functional group, in which the carbonyl is bonded to a hydroxyl group. A single compound often contains several functional groups, particularly in biological organic chemistry. The six-carbon sugar molecules glucose and fructose, for example, contain aldehyde and ketone groups, respectively, and both contain five alcohol groups.
The hormone testosterone, the amino acid phenylalanine, and the glycolysis metabolite dihydroxyacetone phosphate all contain multiple functional groups, as labeled below.
While not in any way a complete list, this section has covered most of the important functional groups that we will encounter in biochemistry. Identify the functional groups other than alkanes in the following organic compounds.
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