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Electrophilic Aromatic Synthesis

By | Organic Chemistry

Electrophilic Aromatic Synthesis ReactionsThere are five key types of electrophilic aromatic synthesis (EAS) that are used in introductory organic chemistry:

  • Halogenation
  • Nitration
  • Sulfonation
  • Friedel-Crafts
  • Alkylation

Halogenation is the process of adding a halogen onto an aromatic ring. In order to attach Bromine or Chlorine efficiently a catalyst is used. A recommended catalyst for this is Iron III Bromide/Chloride (FeBr3/FeCl3; the catalyst matches the halogen being added). Iodine can be added without a catalyst, although Iodine is light sensitive and will detach from the ring if in the presence of intense light or heat.

Nitration adds a nitrite group (NO2) to an aromatic molecule. This is conducted by adding fuming sulfuric acid (H2SO4(f)), fuming nitric acid (HNO3(f)), and nitrite ions (NO2+). Fuming sulfuric acid is a mixture of sulfuric acid and sulfur trioxide. Fuming nitric acid is a combination of nitric acid and a nitrogen-oxygen compound.

Sulfonation is similar to nitration but adds a hydrogen sulfite group (HSO3) onto an aromatic ring. The synthesis is done by applying fuming sulfuric acid and hydrogen sulfite ions (HSO3+).

Friedel-Crafts is a type of reaction that adds a carbon chain to the aromatic compound. The synthesis is conducted by using a halogenated carbon chain of what is desired and a catalyst of AlX3 (X being the halogen used on the carbon chain being added). An important limitation of this reaction is the carbon chain will rearrange to add at the most substituted carbon if possible.Electrophilic Aromatic Synthesis Friedel-Crafts Rearrangement

An example is 1-propyl bromide will create a bond between the secondary carbon and the aromatic, not the primary carbon and aromatic. However, if allyl bromide is used as the halogenated carbon chain in the reaction, it will create the bond to the primary carbon because allyl bromide cannot rearrange.

Alkylation is a type of Friedel-Crafts reaction that adds an acyl group to an aromatic molecule. The process is done by adding the desired acyl group in halogenated form (an acyl halide) and using a catalyst such as AlX3 (X matching the halogen used on the acyl halide).

Electrophilic aromatic synthesis gives a base of additions that can help direct and create more complex molecules with other reactions. How these addition effect further reactions will be covered later on.

Common Aromatic Nomenclature

By | Organic Chemistry

Organic Chemistry - Common Aromatic Nomenclature

Aromatic compounds contain a ring of atoms in which electrons are shared throughout the ring in pi bonds (type of bonds unique to double and triple bonds).  Common aromatic nomenclature is similar to naming cyclic molecules, although there are many common names that are applied to aromatic compounds.

The most used common names are:

 

  • Benzene
  • Phenol
  • Benzyl alcohol
  • Toluene
  • Styrene
  • Aniline
  • Anisole
  • Pyridine
  • Xylene
  • Furan

Benzene is the most basic aromatic compound that consists of six carbons in a ring with alternating double bonds. The alternating double bonds in aromatic molecules may also be denoted with a circle in the middle of the ring.

Phenol is a benzene ring that has one of the hydrogen on the ring replaced with an alcohol group.

Benzyl alcohol has a methyl group attached to the benzene ring and an alcohol group attached to the methyl branch.

Toluene has the base of a benzene ring with a methyl group attached to it.

Styrene has an ethylene (two carbons double bonded to each other) attached to a benzene ring.

Aniline has an ammonia group (R-NH2) attached to a benzene ring.

Anisole is composed of a methyl ester group attached to a benzene ring.

Pyridine consists of a six atom ring with five carbons and one nitrogen that have alternating double bonds between them. Like benzene, a circle may be written in the center of the ring to short hand that there are alternating double bonds.

Xylene is composed of two methyl groups attached to a benzene ring. The position of the second methyl group can be notated with a prefix ortho- (next to each other), meta- (there is one carbon gap in-between the groups), or para- (they are across the ring from each other). This notation can be used on other molecules if there are only two functional groups on the benzene ring base.

Furan is an aromatic compound that is made up of a five atom ring. There are four carbons and one oxygen in the ring. Because of two lone pairs attached to the oxygen, furan can alternate between an aromatic phase (in which the oxygen is sp2 hybridized) and where there are only two sets of carbons double bonded to each other (in which the oxygen is sp3 hybridized).

The aromatic compounds listed above are a small amount of commonly used basic molecules. There is more common aromatic nomenclature that is not listed here and will be labeled and identified if used later on.

Alkynes

By | Organic Chemistry

Organic Chemistry - AlkynesAlkynes, also known as acetylenes, are compounds that contain a carbon to carbon triple bond within the organic molecule. Overall alkynes share the same rules for naming compounds except with a couple minor changes.

First, the compound being named will have a –yne ending instead of the –ene ending found with alkenes or –ane for alkanes. An example of an eight chain carbon with the triple bond between the second and third carbons would be: 2-octyne.

Second, if there is a double bond within the molecule, this will take priority in naming over the triple bond. For a chain that is eight carbons long with a double bond between the second and third carbons and a triple bond between the fifth and sixth carbons would be: (cis/trans)-2-octen-5-yne.

Two things to note from this are:

  • The placement of first (or higher priority bond) can be moved in naming. An example is 2-heptene and hept-2-ene are both the same compound.
  • When there are both double and triple bonds in the compound, the last e on the –ene for the double bond ending is dropped.

Another important piece of information is that triple bonds do not have any type of cis/trans characteristic to them like double bonds do. This is because there is only one thing attached to either side of the triple bond.

Organic Chemistry - Forming AlkynesIn order to create a carbon to carbon triple bond on a molecule, there are some synthesis that can be conducted. The most effective way is to use remove two adjacent halides with two equivalents (Eq) of a strong base. The halides may be vicinal or geminal. Two recommended strong bases are sodium amide (NaNH2) and Lithium diisopropylamide (LDA).

In regards to what can be done with alkynes is much broader. Important reactions that can be conducted are:Organic Chemistry - Alkynes Reactions

  • Forming an organic halide: HX(g) or X2(g) with energy can be applied to an alkyne in one equivalent to form an alkene with halide(s) attached across the bond in the trans configuration. This can be done once more to add another set of halide(s) across the bond. If HX(g) was used both times, then the vicinal halide will be formed.
  • Forming a ketone: To add the double bonded oxygen to the most substituted carbon in the bond sulfuric acid (H2SO4), mercury II sulfate (HgSO4), and water is used in the synthesis. To add the doubled bonded oxygen to the least substituted carbon, first BH3 is add followed by H2O2.
  • Forming an alkene (Dissolving Metal Reaction/Birch Reduction): an alkali metal (lithium, sodium, or potassium) and liquid ammonia (NH3 (L)) is used to reduce the triple bond to a double bond. A majority of the product will be in the trans configuration.

There are many more reactions that can occur to an alkyne group to form other functional groups. More reactions involving alkynes will be covered in different topics.

 

Introductory Reactions

By | Introductory Chemistry

Introductory Chemistry - Neutralization and Precipitation ReactionsIn introductory chemistry there are three key types of reactions that are used:

  • Neutralization
  • Precipitation
  • Reduction-Oxidation (Redox)

Each of these reactions are characterized by the reactants (what is put into the reaction) and the products (what it obtained from the reaction).

In a neutralization reaction, the reactants are an acid and base which produce water. Both acids and bases can be defined in multiple ways. The three most common ways to describe them are by Arrhenius, Bronsted-Lowery, and Lewis definitions.

In the Arrhenius definition, the acid will form H+ ions when placed in water. The base will form OH ions when the molecule is placed in water. Some examples are HCl (hydrochloric acid) will dissociate (break up into ions) in water to form H+ ions and KOH (Potassium hydroxide) will form OH ions in water.

In the Bronsted-Lowery definition an acid is described as a proton (H+) donor and the base is a proton acceptor. The more willing the molecule is to give up the proton means it will be a stronger acid. Examples include HBr (hydrobromic acid) is willing to donate a proton and SO3 (sulfur trioxide) is willing to accept a proton.

In the Lewis definition an acid is willing to receive an electron pair while a base donates an electron pair. This concept expands upon the previous idea of what acids and bases are. An example of a Lewis acid and base is: BCl3 (boron trichloride) can act as an acid to Cl (chloride ion) forming BCl4 (boron tetrachloride).

The most basic formula for a neutralization reaction is an acid (HA) and a base (BOH) react to form H2O and some inert compound (AB; an inert compound will have no effect on the reaction occurring).

In a precipitation reaction two ions in a solution react with each other and create a solid. Often times the solid may still be soluble (have the ability to dissolve in solution) but only in an extremely small amount. The molecules that do not dissolve will precipitate as a solid and fall to the bottom of the container.

A precipitation reaction is often written as A+(aq) reacting with B(aq) to form AB(s). In regards to the arrow(s) of the reaction, there is usually a large forward arrow with a small backward arrow denoting that this reaction can be reversed naturally but will favor the products. This is because an extremely small amount of the solid formed is soluble in the solution. Another note is (aq) stands for aqueous phase which means the ion is dissolved in a solution of water.Introductory Chemistry - Reduction-Oxidation Reaction

The last reaction, reduction-oxidation, involves a change of charge or number of electrons on an atom. Because the reaction must be conservative (the matter put into the reaction must equal the matter coming out of the reaction, including electrons), for every reduction reaction occurring an oxidation reaction is also happening. In order to show the two pieces, redox reaction may be split up into two half reactions. By adding the reduction and oxidation reactions together, the redox reaction is formed. The reason why half reactions are used is due to calculating electrical cell potentials which will be dealt with later on.

 

Covalent Bonds

By | Introductory Chemistry

Introductory Chemistry - Covalent and Ionic BondsThere are two types of covalent bonds in chemistry: Non-polar covalent and polar covalent. Both types of bonds refer to the sharing of valence electrons between two atoms. What separates the groups is the electronegativity values (usually based on the Pauling scale) of the atoms.

Non-polar covalent bonds are the most basic type of bond that will occur between two atoms. In this bond, two electrons are shared between the atoms which have the same electronegativity (desire to take an electron from another atom).  The most common type of non-polar covalent bonds that occur are in the diatomic molecules (molecules that naturally exist in pairs). There are seven diatomic elements:

  • Hydrogen
  • Nitrogen
  • Oxygen
  • Fluorine
  • Chlorine
  • Bromine
  • Iodine

Because each atom in these molecules is identical, there is zero difference in electronegativity. This is why diatomic molecules are non-polar covalent bonded.

Polar covalent bonds do have a difference in electronegativity. This results in a shift in the bond to favor the electrons being closer to one atom. An extreme version of a polar covalent bond is an ionic bond. In an ionic bond the electrons are practically removed from one atom and given to the other. The polar covalent bond is in-between a non-polar covalent and ionic bond. On the Pauling scale, the difference in electronegativity would be between 0 and around 1.5 for polar covalent.

Some examples of molecules with polar covalent bonds are: water, nitric oxide, and carbon dioxide.

Examples of compounds with ionic bonds are: sodium chloride, hydrochloric acid, and magnesium oxide.

Overall, the two types of covalent bonds (non-polar and polar) and ionic bonds are a simple way to explain the characteristics of valence electrons shared between two atoms in a molecule. The key concept to take away from this topic is electronegativity is how the bonds between atoms are characterized. Later on, intermolecular forces will be covered and this is how to describe the interactions between molecules.

 

 

Lewis Structures

By | Introductory Chemistry

Introductory Chemistry - Lewis StructuresLewis structures are a common way to describe the valence electrons and bond configuration of a molecule. In a Lewis structure drawing, there are five key parts that you will need know to understand them:

  • Elements and Atoms
  • Charges
  • Single electrons and lone pairs
  • Bonds
  • Resonance

Each atom within the drawing is described by the letter(s) used on the periodic table. Some examples are: oxygen is denoted by an O, lead is written as Pb, and methane (CH4) is described by a C surrounded by four H’s.

On each of these atoms in the Lewis structure is a charge. This can be found in the top right hand corner of each elemental symbol. The purpose of labeling a charge on each atom is to identify if there are less, the normal amount, or more valence electrons than normal. If there is no charge, it can be left blank or be written with a “0”. The common notation is to leave it blank if there is no charge.

If there is a negative charge on the atom, then it will be written with a minus sign (-) and the number of extra electrons it has. If there is only one extra electron, the charge can be written as “-1” or just “-“.  The same applies to positive charges (fewer electrons that normal are present in an atom’s valence shell), but a positive sign (+) will be used instead of a minus sign.

The valance electrons in a Lewis structure that are unique to an atom are described by a single or pair of dots. Each pair of dots in a drawing is referred to as a lone pair because it is denoting two electrons of opposite spin in the same orbital. They are not directly interacting with any of the other atoms in the structure. If there is only one set of one electron (represented by a single dot) in the whole molecule, then the molecule can be referred to as a radical. These are usually reactive compounds.

Bonds between atoms in a Lewis structure are represented by one, two, or three lines between atoms. These are for single, double, and triple bonds, respectfully. Four bonds between atoms do not typically occur and are involved in complexes with transition metals. The purpose of atoms bonding is so they may complete the octet rule (the tendency to achieve 8 valence electrons to have a stable outer shell). Each atom contributes one electron in each bond which counts as two towards the octet rule for both atoms.

Some common exceptions to this rule are:

  • Hydrogen prefers to have 2 electrons in total
  • Boron is stable with 6 electrons overall
  • Sulfur can have 10 or 12 electrons in a drawing because it can have an expanded octet from the d orbital

Larger elements can also use the expanded octet (the ability to have more than eight valence electrons). But for the purposes of introductory chemistry, Lewis structures will usually use these exceptions.

Resonance is the concept that a molecule can have multiple forms with the atoms having various charges. The molecule will shift between these forms constantly which results in a combination of these possibilities being the realistic molecule.

An example is SO32-. In one drawing, two of the oxygens will be given a negative charge while the other one will have a neutral charge. In another drawing only one oxygen will have a negative charge and the sulfur will have a negative charge.

Overall Lewis structures are a simple way to describe a molecule in regards to its electrons. These drawings can show potential shifts of charge which are used in explaining some later reactions.

Alkenes

By | Organic Chemistry

Organic Chemistry - AlkenesAlkenes are a slightly more complicated type of molecule than alkanes. What classifies a molecule as an alkene is the presence of at least one carbon to carbon double bond. When describing the nomenclature of these molecules, the naming system is similar to alkanes with a few additions.

First, the double bond(s) will be denoted by a number before the longest chain name in addition to an -ene ending. The first carbon in a double bond that gives the lowest number will be used in the nomenclature. If there are multiple double bonds in the chain, then an a + prefix to clarify how many there are will be added before the –ene ending. Some examples are: 2-butene and 1,3-butadiene.

Second, if there are any smaller chains attached to the longest chain which contains the carbon to carbon double bond(s), then the first double bond that is labeled is the one which is the most substituted. The most substituted is defined as the carbon to carbon double bond which has the most carbons attached to it. An example is 2-methyl-1,5-hexadiene.

Third, when describing which way the largest branches (chains with the largest combined molecular weight) on either side of the double bond(s) are positioned, there are two ways to denote this. If they are on the same sides as each other, then either z– (for Zusammer) or cis– is added to the beginning of the nomenclature. If they are on opposite sides then either e– (for Entgegen) or trans– is added to the beginning of the nomenclature.

If there are multiple double bonds in the compound then all the double bonds’ configurations are labeled at the beginning in their priority and are separated by commas. Some examples are: cis,cis-2,4-hexadiene and Z,E-2,4-hexadiene. (Note: if there are any small chains added onto the molecule’s main chain, they are labeled in between the cis/trans/z/e and the numbers involving the double bond(s)).

In regards to an alkene’s reactivity, there are several reactions that can be used to create an alkene or add upon the molecule. Here are two basic examples of creating an alkene are:

First, take an alkane which has a halide on it. Depending on where you want the double will determine the second step:Organic Chemistry - Creating Alkenes Reactions

  • If you want the double bond to go to the least substituted side, then a hydroxide like potassium hydroxide (KOH), ethanol (C2H5OH or EtOH), and heat will be applied to the molecule.
  • If you want to double bond to go to the most substituted side, then t-butoxide (tBuO) will be applied to the molecule.

By applying sulfuric acid (H2SO4) to an alcohol group on an alkane you can produce a most substituted double bond. This is another common method to produce an alkene similar to the first reaction but with a different starting material.

Organic Chemistry - Alkenes ReactionsSome reactions that use alkenes or dienes to create a new product are:

  • Adding a least substituted alcohol group (hydroboration): First, apply BH3. Second, apply H2O2 and OH.
  • Adding a most substituted alcohol group: First, apply mercury (II) acetate (Hg(OAc)2 ), acetic acid, and water to the molecule. Second, add sodium borohydride (NaBH4). R-OH can be substituted for water to add –OR instead of –OH to the molecule. R being any size or type of carbon chain.
  • Adding one or two halogens: If adding one halogen, then apply HX in the gas phase with heat. The halogen will add to the most substituted carbon. If adding two halogens, then apply X2 in the gas phase with heat. The two halogens will add one to each of the carbons that were involved in the double bond.

These are only a handful of reactions that alkenes have to offer. Much more will be covered later on, but these are the basics that need to be memorized.

Periodic Table

By | Introductory Chemistry

Introductory Chemistry - periodic table of elementsThe periodic table is a chemist’s tool for referring to trends and basic information about an atom. The most common type that is used is a condensed version. This is done by moving the lanthanides (elements 57 through 71) and actinides (elements 89 through 103) to the bottom of the page. The reason why the extended periodic table is not as common is because it is easier to fit the condensed version on paper.

Currently, there are 118 elements in the periodic table. All elements up to number 92 (Uranium), excluding numbers 43 and 61 (Technetium and Neodymium), are naturally occurring elements. This means that these elements can be found in nature in some amount. Element 93 (Neptunium) and above, 43, and 61, are all artificially made in labs through applications of nuclear chemistry.

The first general trend is elements within the same column tend to share similar properties. This is because elements in the same column have like outer electron configurations. Therefore, they react to other elements in similar manners.  The common names that you need to know for introductory chemistry are:

  • Column 1 is the alkali metals
  • Column 2 holds the alkaline earth metals
  • Columns 3 through 12 contains transition metals
  • Column 17 elements are referred to as halides
  • Column 18 represents noble gases

The alkali metals are reactive due to having one electron in their outer shell. Because elements prefer to have a full electron shell, the alkali metals are willing to give up the electron to another atom. Going down this column the elements become more reactive because the outmost electron is further away from the nucleus; where the positive charge of the atom is.

The electron, which is negative, is attracted to the nucleus due to opposite charges. As the distance between the two increases, the force holding them close to each other weakens. In addition to the distance, the inner electrons also repel the outer electrons and create an effect called shielding (where inner electrons weaken the nucleus’ attraction on outer electrons). This is why larger atoms are more willing to give up electrons in their column. As a side note, hydrogen is not a metal. But it does share similar properties because it only has one outer electron.

The alkaline earth metals are similar to the alkali metals in how they are willing to give up electrons in order to have a full outer shell. Although, instead of only one outer electron, the alkaline earth metals have two.

Elements in columns 3 through 12, the transition metals, have more complicated properties. One of the notable properties is the ability to create complexes in solutions because they have the d orbital as one of their outer shells. In introductory chemistry, the transition metals are not heavily explored. This is expanded upon in inorganic chemistry. The key part of transition metals is being able to understand their electron configuration and recognizing them.

On the opposite side of the periodic table, the halides have seven electrons in their outer shell. Because they are one away from having a full outer shell, they are very reactive and willing to accept an electron. Although as you go down column 17, the halides become less willing to accept an electron due to the distance of the outer shell and shielding. Referring back to hydrogen, this element is only one electron away from having a full shell. Because of this, hydrogen can be forced to accept an electron under certain conditions, although it will usually give it away.

The last column that is often referred to in introductory chemistry is the noble gases. These elements have full outer shells that cause them to be unreactive. Under certain extreme conditions, noble gases can be forced to react with other elements. But typically they will remain as a single atom.

There are a lot more pieces of information that can be extracted from the periodic table. These tools will be covered later on in the topics they are used in.

Alkanes

By | Organic Chemistry

Alkanes - Organic ChemistryAlkanes are the simplest type of organic compound. They are composed of chains of carbons single bonded to each other and hydrogens. When naming an alkane molecule, the longest chain of carbons is used to derive its nomenclature. Most carbon chains that basic organic chemistry works with do not go beyond eight. For reference, here is a list of alkane compounds from 1 to 10 carbon chains:

  • Methane
  • Ethane
  • Propane
  • Butane
  • Pentane
  • Hexane
  • Heptane
  • Octane
  • Nonane
  • Decane

A compound can still be an alkane even if there are smaller chains attached to the longest chain. These additions are included in the name so that other chemists may be able to draw the molecule by its nomenclature if necessary. The smaller chains will keep the base name as shown above (two carbon chain has a base eth-; five carbon chain has a base pent-), but the ending will be changed to a –yl instead of –ane.

In order to communicate where this smaller chain is at, a number will be placed before the name of the smaller chain carbon denoting which carbon it is attached to on the longest chain. An example of a three carbon chain attached to the fifth carbon on the longest chain is: 5-propyl.

If multiple small chain carbons of the same length are attached to the long chain carbon, then the each number will be separated by a comma and ordered from lowest to highest. This is followed by a hyphen and same naming system as before in addition to a prefix to describe how many groups are attached. An example of 3 single carbons attached to the second, third, and fifth carbon on the longest chain is: 2,3,5-trimethyl.

When ordering the small chain carbons, they will be placed alphabetically by their base name. In addition to this, numbers and letters must be separated by hyphens. For example, a 2 carbon chain on the sixth carbon and a 3 carbon chain on the second carbon of the longest chain would be: 2-propyl-6-ethyl.

The final piece of naming an alkane is the longest chain carbon will have a base describing how many carbons are in it with an ending –ane. Taking this name, attach it to the end of the smaller named carbon chains. An example is a single carbon on the third carbon and a two carbon chain on the fifth carbon of an eight long main chain: 5-ethyl-3-methyloctane.

Alkanes and Free Radical Reaction - Organic ChemistryAs the compounds get more complicated with the addition of other atoms, the nomenclature gets minor additions to it. But the main concepts here are always applied. These rules for naming alkanes may seem overwhelming when seeing them for the first time. But, all it takes is practice to master this material as well was any other piece of organic chemistry.

Going beyond the nomenclature of alkanes, because they are a fairly neutral type of organic compound, there are very few reactions that will occur. One reaction that can be prompted with some energy is a free radical reaction. By applying energy to a halogen and alkane, there is the possibility that a radical halogen atom will replace a hydrogen on the alkane to form an organic halide. This process will be covered more in depth later on.