A primary carbon is a carbon atom which is bound to only one other carbon atom. It is thus at the end of a carbon chain. In case of an alkane, three hydrogen atoms are bound to a primary carbon (see propane in the figure on the right). A hydrogen atom could also be replaced by a hydroxy group, which would make the molecule a primary alcohol. As part of an individual’s “primary” carbon footprint, representing the emissions over which an individual has direct control. The remainder of an individual’s carbon footprint is called the “secondary” carbon footprint, representing carbon emissions associated with the consumption of goods and services.
The nomenclature is a very important part of organic chemistry. The names are not given only to compounds but also to the carbon atoms that make up this compound.
Thus, we can classify carbon atoms as primary, secondary, tertiary, or quaternary. These terms refer to the substitution level that a given carbon has in a molecule. In other words, these terms are used to describe how many other carbons a given carbon is attached to. This classification applies only to saturated carbons.
- Primary (1°) carbon atom – bonded to one other carbon atom,
- Secondary (2°) carbon atom – bonded to two other carbon atoms,
- Tertiary (3°) carbon atom – bonded to three other carbon atoms,
- Quaternary (4°) carbon atom – bonded to four other carbon atoms.
This can be explained by one of the important properties of carbon and is its tetravalency. Carbon is a strict octet follower, which means it needs a maximum of 8 electrons to form stable compounds. Since a carbon atom has 4 valence electrons, it can form up to 4 bonds with different elements. Part of the reason why there are millions of compounds of carbon is its ability to form a very stable bond with another carbon atom.
The same terminology is used for carbocations. A primary carbocation is attached to one other carbon, a secondary to two, and a tertiary to three. A quaternary carbocation does not exist without violating the octet rule.
For example, you get the following compound to determine which primary, secondary, tertiary, or quaternary carbons are. As mentioned above, a primary is attached to one carbon atom, a secondary to two, a tertiary to three, and a quaternary to four other carbon atoms. For each carbon atom, you need to count how many carbon atoms next to it that particular carbon atom is connected to.
There is another rule:
- Primary carbon atoms are always at the end;
- Secondary carbon atoms are in the middle (between two other carbon atoms);
- Tertiary carbon atoms are branched out in three different ways;
- Quaternary carbon atoms have the most carbon atoms around (max 4).
OK. These are carbon atoms. But what about the hydrogen atoms which are bonded to these carbon atoms? Yes, they can also be primary, secondary, and tertiary. It depends on the carbon atoms they are attached to. So follow the next rule for hydrogens:
- Primary hydrogen atoms are attached to primary carbon atoms;
- Secondary hydrogen atoms are attached to secondary carbon atoms;
- Tertiary hydrogen atoms are attached to tertiary carbon atoms.
In our example, we have a total of 18 primary hydrogens. Because each primary carbon has 3 hydrogen atoms, and we have 6. Secondary hydrogen atoms have a total of 4 (2 hydrogens per secondary carbon atom), and tertiary 2 (1 hydrogen per tertiary carbon atom).
Let’s go back to the carbons. Let’s look at what are called carbons that are bonded to other atoms and atomic groups such as halides, hydroxides, amines.
Alkyl halides
It should also count here how much carbon atoms are attached to a particular carbon. Halides (fluorine, chlorine, bromine, or iodine) are not counted. Thus, the primary alkyl halide is one that has only one carbon atom bound to itself. The secondary has two carbon atoms and a halide, and the tertiary has three carbon atoms and a halide bonded to itself. The quaternary alkyl halides don’t exist because that would involve breaking the octet rule.
Alcohols
The rules apply the same way for alcohols as it does for alkyl halides. For the most groups like alcohols, alkyl halides, and hydrogen atoms to determine if it’s primary, secondary, or tertiary, look at the carbon atom that bears those atoms, ignore this atom or group and count how many carbons are attached to it.
Amines
Here is a slightly different story. Amines are named according to the number of carbons attached to nitrogen. Primary, secondary, and tertiary amines are nitrogens bound to one, two and three carbons, respectively. They also form quaternary amines, since the nitrogen has a lone pair and it possible to form another bond to carbon. They bear a positive charge on nitrogen and are not at all basic. They are often referred to as quaternary ammonium salts.
Numbers of carbon atoms attached to carbon atoms also govern how they will react.
For carbocations, that is cations if carbons, carbons with more carbons attached on (i.e. tertiary) tend to be less electron-deficient due to hyperconjugation from nearby C-H bonds. Therefore, tertiary carbocations are more stable compared to secondary, primary, and methyl, respectively.
Another case is that of alcohols. Primary alcohols can be oxidized to aldehydes and carboxylic acids (two levels). Secondary alcohols can go only one level of ketones, and tertiary alcohols cannot be oxidized at all.
Main Difference – Primary vs Secondary Alcohol
Monohydric alcohols are classified into three types: primary alcohols, secondary alcohols, and tertiary alcohols. This article focuses on the difference between primary and secondary alcohol. The main difference between primary and secondary alcohols is that the hydroxyl group of a primary alcohol is attached to a primary carbon, whereas the hydroxyl group of a secondary alcohol is attached to a secondary carbon atom. There are various tests to distinguish between primary and secondary alcohol. Such methods include oxidation test, reaction with hot reduced copper, Lucas test, and Victor Meyer’s method.
This article examines,
1. Classification of Alcohols
2. What is Primary Alcohol
– Characteristics, Structure, Properties
3. What is Secondary Alcohol
– Characteristics, Structure, Properties
4. What is the difference between Primary and Secondary Alcohol
Classification of Alcohols
Alcohols are a type of hydrocarbon in which the hydroxyl (-OH) functional group is bonded to a sp3 hybridized carbon of an alkyl group. As per the definition of alcohols, the hydroxyl group must be bonded to a carbon atom of the main chain, which is indicated by the position number. In general, the simple alcohols are named by their alkyl group, followed by the word alcohol; for example, ethyl alcohol. These hydrocarbons are polar compounds due to the presence of C-O and O-H bonds. Because of the unshared electrons on the oxygen atom, alcohols can form oxonium ion (ROH2+) by reacting with a strong acid. Hence, alcohols are known as weak Lewis bases. An alcohol can form hydrogen bonds with water and another alcohol molecule. Alcohols are weak acids with 16 to 18 pKa values. Alcohols react with halogen acids such as HCl, HBr, and HI and convert into alkyl halides. Monohydric alcohols consist of one hydroxyl group, e.g., CH3OH, C2H5OH, etc. Alcohols with two hydroxyl groups are called dihydric alcohols, e.g., CH2OH-CH2OH (ethylene glycol). Based on the type of carbon atom to which the hydroxyl group is attached, monohydric alcohols are classified into three types, namely; primary alcohols, secondary alcohols, and tertiary alcohols.
What is Primary Alcohol?
Primary alcohols are the alcohols in which hydroxyl group is attached to the primary carbon of the alcohol molecule. This is the simplest form of alcohol. Primary alcohols can be oxidized to form aldehydes (RCHO) under mild conditions. However, when stronger oxidants are used, primary alcohols first transform into aldehydes and then rapidly transform into carboxylic acids (RCOOH). The reactivity of primary alcohol is less than that of a secondary alcohol. In Lucas test, primary alcohols do not form oily layers, unlike secondary and tertiary alcohols. However, they form volatile alkyl halides upon heating. Examples of primary alcohol include methanol, ethanol, propanol, etc.
Figure: Methanol, an example of primary alcohol.
What is Secondary Alcohol?
Secondary alcohols are the alcohols in which hydroxyl group is attached to the secondary carbon atom of the alcohol molecule. Secondary alcohols are converted into ketones (R2CO) upon oxidation under mild condition. Stronger oxidants, however, lead to the formation of acids or mixture of acids. With Lucas test, secondary alcohols form oily layers within about five minutes. Secondary alcohols are more stable and more reactive than primary alcohols. However, secondary alcohols are less acidic than primary alcohols. Some examples of secondary alcohol include isopropyl alcohol and butyl alcohol.
Primary Carbon Bonds
Difference Between Primary and Secondary Alcohol
Definition
Primary Alcohols:Primary alcohol is an alcohol which has the hydroxyl group connected to a primary carbon atom.
Secondary Alcohols:Secondary alcohol is an alcohol which has the hydroxyl group on a secondary carbon atom.
Reactivity
Primary Alcohols: Primary alcohols are less reactive than secondary alcohols.
Secondary Alcohols: Secondary alcohols are more reactive than primary alcohols.
Esterification
Primary Alcohols:Rate of esterification is highest in primary alcohols.
Secondary Alcohols:Rate of esterification is less in secondary alcohols.
Easiness of Dehydration
Primary Alcohols:Primary alcohols are difficult to dehydrate than secondary alcohols.
Secondary Alcohols:Secondary alcohols are easier to dehydrate.
Dehydrogenation
Primary Alcohols:Primary alcohols result in aldehydes and hydrogen.
Secondary Alcohols: Secondary alcohols results in ketones and hydrogen.
Lucas Test
Primary Alcohols:Primary alcohols do not form oily layers.
Secondary Alcohols:Primary alcohols form oily layers within about five minutes.
Stability
Primary Alcohols:Primary alcohols are less stable than secondary alcohols.
Secondary Alcohols:Secondary alcohols are more stable.
Victor Meyer’s Test
Primary Alcohols: Primary alcohols give a blood red color.
Secondary Alcohols:Secondary alcohols give a blue color.
Primary Carbonation
Acidity
Primary Carbon Substrate
Primary Alcohols:Primary alcohols are more acidic than secondary alcohols.
Secondary Alcohols:Secondary alcohols are less acidic.
Summary
Primary and secondary alcohols are two types of alcohols coming under monohydric alcohols. In primary alcohols, -C-OH group is attached one carbon atom, whereas, in secondary alcohols, -C-OH group is attached to another two carbon atoms. Based on this main difference between primary and secondary alcohols, their properties vary. Lucas test and Victor Meyer’s test are used to identifying primary and secondary alcohols.
Primary Carbon Secondary Carbon
Reference:
1.Johnson, A. W. (1999). Invitation to organic chemistry. Jones & Bartlett Learning.
2.Madan, G. (2005). S. Chands Success Guide (Q&A) Inorganic Chemistry. S. Chand Publishing.
3.Competition Science Vision, 2003, Vol.5, Pratiyogita Darpan Group, India
Secondary Carbons
Image Courtesy:
1. “Methanol flat structure” By Cacycle – Own work (Public Domain) via Commons Wikimedia
2. “Secondary alcohol” By Skend – Own work (Public Domain) via Commons Wikimedia