woocommerce checkout page call action when place order is trigger

functions.php

add_action('woocommerce_after_checkout_validation', 'rei_after_checkout_validation');

function rei_after_checkout_validation( $posted ) {

    // do all your logics here...
    // adding wc_add_notice with a second parameter of "error" will stop the form...
    // wc_add_notice( __( "OMG! You're not human!", 'woocommerce' ), 'error' );

    if (empty($_POST['captcha'])) {
         wc_add_notice( __( "Captcha is empty!", 'woocommerce' ), 'error' );
    }

}

Structure $posted array =>
Array
(
    [terms] => 0
    [createaccount] => 0
    [payment_method] => cheque
    [shipping_method] => 
    [ship_to_different_address] => 
    [billing_first_name] => 
    [billing_last_name] => 
    [billing_company] => 
    [billing_email] => [email protected]
    [billing_phone] => 
    [billing_country] => PH
    [billing_address_1] => 
    [billing_address_2] => 
    [billing_city] => 
    [billing_state] => 
    [billing_postcode] => 
    [order_comments] => 
)